Some cash games, especially with blinds, require a new player to post when joining a game already in progress. Posting in this context means putting an amount equal to the big blind or the minimum bet into the pot before the deal. The post is a "live" bet, meaning that the amount can be applied towards a call or raise when it is the player's turn to act.
A player who is away from his seat and misses one or more blinds is also required to post to reenter the game. In this case, the amount to be posted is the amount of the big or small blind, or both, at the time the player missed them. If both must be posted immediately upon return, the big blind amount is "live", but the small blind amount is "dead", meaning that it cannot be considered in determining a call or raise amount by that player. Some house rules allow posting one blind per hand, largest first, meaning all posts of missed blinds are live.
Posting is usualy not required if the player who would otherwise post happens to be in the big blind. This is because the advantage that would otherwise be gained by missing the blind, that of playing several hands before having to pay blinds, is not the case in this situation. It is therefore common for a new player to lock up a seat and then wait several hands before joining a table, or for a returning player to sit out several hands until the big blind comes back around, so that he may enter in the big blind and avoid paying the post. For this same reason, only one set of missed blinds can be accumulated by the player; old missed blinds are removed when the big blind returns to that player's seat because the player was never in any position to gain from missing the blinds.