Above is a diagram of how bonus symbols may fall on a 5 reel slot machine; these bonus symbols will trigger a bonus feature.
A bonus symbol is a special symbol that triggers a bonus feature; this could be a bonus game or free spins or any other 'bonus feature' be that a gamble game or a second set of reels. Depending on the rules of that specific slot game the bonus symbol will trigger the bonus feature when one or more appears on the reels.
You may find that bonus symbols often double as scatter symbols so if this is the case you need a certain number of these symbols to trigger the bonus feature and they can appear anywhere on the reels. It shuold be noted that this is not always the case and that often you will need a certain number of bonus symbols on an activated payline for them to activate what ever bonus feature it is that it activates on that specific slot machine. To find out exaclty how a bonus symbol acts on any specific game check the slot machines paytable where you will often find a rules section.