A "bouquet" is a bunch of flowers that have been gathered together and arranged to look beautiful.
There's a wedding tradition in some cultures called "the bouquet toss". At the wedding reception after a couple gets married, all of the unmarried women are gathered together. The bride turns away from them and throws her bouquet of flowers over her shoulder. The woman who catches the bouquet is supposed to be the one to get married next.
The word "toss", by the way, means to throw something lightly.