Before cards are dealt in a game, there needs to be some initial money in the pot.

The player to the immediate left of the button is first to receive a card and is required to post a “small blind”.  In a fixed limit game, the small blind is equal to half the lower limit bet.  The player to the immediate left of the small blind is required to post the “big blind”, equal to the lower limit bet.  These blinds count toward your total investment toward the first round of betting.  So if the pot isn't raised pre-flop, the small blind will only have to put in another half a bet to call.  Likewise, the big blind won't have to put anything more in the pot, although the big blind will have the choice to raise their own blind.

When a new player sits down at a game, they must either post the equivalent of the big blind or wait for their turn on the big blind.  This prevents players from gaining an advantage by avoiding the blinds and jumping in and out of games.  You can also wait for the blind to get to your position before joining.  This is called coming in on the blind.