A ready check is an automatic survey that can initiated by a party or raid leader. Anyone promoted to raid assistant by the leader is also able to use the command. The command is initiated by typing /readycheck, or by clicking the "Ready Check" button in the raid ui panel.
All members of the raid will receive a popup window that asks if the player is ready. If all players click "yes", the person who initiated the check will receive a message indicating so. If a player clicks "no" or does not respond within the prescribed time limit a message will be sent to the leader indicating that they are either not ready or AFK, respectively. Players who are flagged with the <AFK> flag will auto-respond as being AFK.
Ready checks are most often used in preparation for a boss encounter to make sure everyone is ready to commence. They are also often used as a means to survey which players have returned if the raid takes a break for any reason.
Note: The ready check popup window will not appear for group members who are currently playing in a Battleground. This can sometimes make a raid leader (especially when organizing a Premade) erroneously believe that a team member is AFK.