Multiplier symbols as the name suggests can multiply your winnings, for this reason they are very popular among slot machine players. They increase the payout of a winning combination by their given multiplier amount; this will be on the slots paytable.
In the diagram above you can see how 1 multiplier symbol has fallen in a winning combination. The winning combination in this case will be muliplied x3.
For example, in a classic slot you may get 100 coins for a combination of 3 bar symbols on the slots payline, however, if there is a multiplier symbol available, say for example a cherry symbol, this will substitute for the bar symbols and if the multiplier value of the cherry symbol is x 3, the winnings will be tripled. This means that the player would win a total of 300 coins.
This is why slot machines with multiplier symbols are a real plus for players.