Bonus symbols are symbols you find in a slot machines and which trigger a bonus feature such as a bonus game, free spins or any other bonus feature.
Usually you need a certain number of bonus symbols to land on the reels in a specific order to trigger a bonus game, but that depends on the game you are playing. The best thing to find out, is to read the game rules, since it can differ from game to game.
In video slots games, you usually need 3 or more bonus symbols on an active payline from left to right to trigger a bonus round, and sometimes it might be enough to get a number of them visible on the reels.
Sometimes, the bonus amount can be multiplied depending if the slot has a multiplier or not. In some slots, you can multiply your winning by 2 times, 3 time or even more, especially in free spins rounds where you can multiply your winnings with 20 times, 30 times or even more.
You can find many slots in online casinos that offer bonus symbols in their slots, and you will always have a big selection of these types of slots playing them for real money and try your luck in. The game developer Net Entertainment is one of the best developer of slots which offers bonus symbols.
Just keep your eyes open for Net Entertainment games and you will surely come across a casino slot game which offers some nice bonus symbols.