Multipliers are special symbols - not always part of the main game area - that increase the value of the prize on a winning play by multiplying it by a value that will be indicated on the symbol in question.
For example, if a player bets R$1 on a spin and the online slot has a 2x multiplier, the player would win R$2. If the multiplier was 3x, the player would win R$3.
The most common multipliers will double, triple, or quadruple the prize. However, some games feature much higher multipliers, which can even be progressive, meaning that they get higher and higher with each new winning spin or activated pay lines.