Posting is the action taken by a player when they place a bet onto the table prior to the dealing of the cards. Any bet placed prior to the deal is said to be "posted".
In many casinos, a player who is just joining a table is required to post a big blind even when they are not in the position to be the big blind in order to be dealt a hand. This type of posting is used to prevent players from table-hopping between one table and the next in a casino, always skipping out just prior to having to pay a blind. Players who are joining a table generally have the option to either post a big blind to get a hand immediately, or wait until their seat becomes the big blind, and join the game then. If they do the latter, they need only pay the normal big blind prior to the deal - they do not have to post any additional money.
In most casinos, a player who changes seats at a table in such a fashion that they are moving away from the button (in other words, they will have more hands to come before becoming the big blind) must also post a big blind in order to be dealt a hand, or wait an equivalent number of hands so that they are the same number of seats away from the button as they were before they moved. This is to avoid players using seat changes at a single table to avoid the blinds.
In some smaller casinos, new players are not required to post a blind and may be dealt into the game immediately. The casino employees merely pay attention to such players to ensure that they don't try to use seat changes or table changes to avoid paying the blinds.