Why do we have discussions about math?
The richest math learning occurs when students engage in a discussion about their math work and learning, supported and facilitated by you, the teacher. A math discussion about a variety of student work  not just one "teacherapproved" method  deepens students' mathematical understanding in the following ways:

Provides opportunities for students to communicate and reason about math ideas, strategies, concepts, and representations.

Makes students' misconceptions clear.

Enables every student in the class to develop an understanding of key mathematical concepts.

Develops the habits of mind called for by the Standards for Mathematical Practice.

Builds students' own identities as mathematicians when they see their own and their peers’ ideas displayed and argued for and against.

Provides entry points for students who are struggling to access and understand the lesson goal.
How do you facilitate a math discussion?
1. Use the prompts to prepare
Each lesson has suggested topics and prompts.
Conceptual Understanding and Application Lessons:

Each lesson has "topics" you can include in the discussion.
 Each Task Implementation Guide has questions and "prompts for discussion", which you can use to make decisions about the discussion of the work.
Fluency/Procedural Skills Lessons:

Each lesson has a question about an insight students should gain that you can discuss.
 Each Answer Key has notes on what to "look for" in student responses, which you can use to make decisions about the discussion of the work.
2. Select student work to discuss
Through the choice of work to discuss, you are able to focus the lesson on the big ideas you want students to grapple with and, ultimately, to understand.
The Task Implementation Guide (Conceptual Understanding and Application lessons) and Answer Key (Fluency/Procedural Skills lessons) have sample student work and answers you can discuss. You can also use these as guidance in choosing work from your own students to discuss.
You will want to preplan the ideas you want to discuss and the order you want to discuss them in. You'll then be able to look for these ideas as you circulate, observe, and ask questions during the student work time.
In general, three pieces of student work are adequate to support a rich discussion.
3. Introduce and discuss the work
When your class is learning how to discuss work, you should introduce and explain each piece of work.
As your students learn how to engage in a math discussion, they can learn how to introduce and explain their own work as their peers listen and respond. Teacher language can become student language!
Other Considerations

How will students sit so they can see the work and participate in the discussion?

How will you record new learning and display visual supports?

What mathematical vocabulary words will you be sure to highlight during the discussion?
 How will students share mathematical models and diagrams to support their arguments and explanations?
For more information on supporting English Language Learners and other special populations, see this document.
Language for math discussions
Students 
Teacher 
Share explanations of their problemsolving strategies: 1) Getting started:
2) If they did something similar to or different from another student:
3) If they support the argument of another student:
4) If they want to critique the argument of another student:
5) If they learn another way:

Help students determine for themselves whether something is mathematically correct:
Revoice:
Ask students to restate someone else's reasoning:
Apply their own reasoning to someone else's:
Encourage further participation:
Wait time:
Respond neutrally to errors:
Give students a lens for speaking and listening:

Additional Reading:
De Garcia, L. A. (n.d.). How to Get Students Talking! Generating Math Talk That Supports Math Learning. Math Solutions. Retrieved from http://www.mathsolutions.com/documents/how_to_get_students_talking.pdf
Smith, M. S., & Stein, M. K. (2011). 5 Practices for Orchestrating Productive Mathematics Discussions. Reston, VA: National Council of Teachers of Mathematics.
Stein, M. K., Engle, R. A., Smith, M. S., & Hughes, E. K. (2008). Orchestrating productive mathematical discussions: Five practices for helping teachers move beyond show and tell. Mathematical Thinking and Learning, 10(4), 313–340.
Webb, N. M., Franke, M. L., Ing, M., Wong, J., Fernandez, C. H., Shin, N., & Turrou, A. C. (2014). Engaging with others’ mathematical ideas: Interrelationships among student participation, teachers’ instructional practices, and learning. International Journal of Educational Research, 63, 79–93. http://doi.org/10.1016/j.ijer.2013.02.001