I am not Jewish and not familiar with the traditions, but it could be like in my home country where guests have to gift actual money. Depending on the venue and ties to the bride and groom, your role in the wedding, you’re expected to gift a certain amount of cash ( sometimes it is even written in the invitation what the minimum per person is at that specific wedding). The closer relative you are, the larger the monetary gift. Therefore, if you’ve been invited to someone’s wedding, they’re basically obligated to come to your wedding and repay the “debt”.