A commit line is a line on a poker table which forms an inner oval or circle area on the table which is considered the "live betting area". Chips placed within the inner oval are considered to have been bet; chips outside the line are not. This provides clear indication of when a player is betting versus when they are merely "staging their chips" (building the stack they wish to bet) or playing with their chips. The commit line can also be used to indicate when a player folds - pushing one's cards over the commit line is generally interpreted as a fold, as opposed to merely moving the cards around.
Tables without commit lines generally use motion to determine whether a player is betting or not, with the declaration that a forward motion of chips counts as a bet, where a side or back motion of chips does not. Tables with commit lines may also use motion to determine whether a player has bet or not, if the floorman determines it is "in the interest of the game" to do so.
Some poker rooms are more picky than others about the rules that determine what a "bet" is, but in general, when using tables with commit lines, any placement of chips beyond the line (into the center oval) counts as a bet.