Hindu believes life consists of 5 basic elements which are water, earth, sky, fire, and wind. So, in the Hindu wedding, there are 5 ceremonies worshiping these elements.
At the end, when groom is supposed to give sacred neclace to bride both of them walk around this fire for 7 times('cause in earlier days only 7 planets in solar system were considered) and Fire represents Sun source of energy. It's like keeping all of them as wintnesses and then they promise to support each other forever and ask for their blessings in the journey thereafter.