Well, basically the call is the amount of the bet on the table.
For example, let's say 4 players are in a hand. 1st to act bets T100. 3rd to act calls out of turn which means they have only called T100. Now, 2nd to act still has to decide what they are going to do. If 2nd to act calls or folds, then 3rd to act can only call. If 2nd to act rasises, 3rd to act can either call the raise or fold, however the player should lose their call (the T100) if they fold.
In the case of an all-in by the 1st to act, the call is the amount of the all-in. In the example above, if the 2nd to act had gone all-in and it was for more than the bet already on the table, 3rd to act can fold but must still forfeit the amount of the call.
Hopefully I didn't ramble too much and that made sense.