As the expression suggests, fixed limit poker is a type of poker where betting limits are fixed, i.e., players don’t have any options in terms of determining bet sizes.
In no-limit variations, you can bet as much as you like at any point, up to the amount of chips you have in front of you. In pot-limit, you’re only limited by the size of the pot.
In fixed-limit poker, however, the bet size always remains the same regardless of how much money players have or how big the pot is.
For example, in a $2/$4 Limit Hold’em game, there is a “small bet” and a “big bet.” The small bet is $2, and it is used before the flop and the flop. The big bet is $4, and it comes into play on turn and river.
So, before the flop, you have the option to call the $2 or raise to $4 (double the small bet), and that’s it. The next player to act can call your raise or raise to $6 (i.e., add another small bet). The number of raises per round is usually limited to three, so one more player can raise to $8, after which all other players can only call or fold.
When the big bet comes into play, the process is the same, but the betting now begins with $4 and raises increases in $4 increments, i.e. $4 to $8, $8 to $12, and $12 to $16, hitting the maximum number of raises.
In some fixed-limit games, the cap on the number of raises is removed once a hand gets heads-up, i.e., only two players are remaining.