Planning Poker, also called Scrum Poker, is a consensus-based, gasified technique for estimating effort or relative size of development goals in software development.
Planning Poker 101 has 3 different variations: Standard, Fibonacci, and T-Shirt sizes. You have the ability to make a selection and have it hidden until the rest of the team is ready to display. This will prevent adding a cognitive bias of anchoring, where the first number shown influences other members estimations. When the team is ready - simply tap on your selection to reveal it.
If you are new to Planning Poker - check out the User Guide and Reading Material that can be found inside the application. This will help you get started and up to speed before using the application for team planning events.