Wagering requirement is a requirement that needs to be met before winnings from a cash bonus such as welcome bonus can be withdrawn.
It is basically an amount you as a player need to bet in order to be able to withdraw your bonus winnings. Some bonuses would require 35X wagering before withdrawing and some 40X depending on the casino.
So, if you deposit 20EUR and claim 100% bonus you get 20EUR bonus money as well. In total, you have 20EUR real money and 20EUR bonus money. If the wagering is 35X then you would need to wager 20EUR Bonus X 35 = 700EUR. The amount of bets before withdrawing should therefore equal 700EUR before withdrawing the winnings. That is what a wagering requirement is.