The best man may escort one of the mothers or grandmothers in to be seated before the ceremony begins.
He may be asked to light candles on an altar, carry the ring his friend is giving in the ceremony, and offer one of the first toasts at the wedding reception. The best man isn't just another attendant.
With some couples the casual approach is comfortable and acceptable. I often joke at the wedding rehearsal that the best man's main job is to catch the groom if he faints. But many couples may secretly hope that the best man in their ceremony will do a little research and create a stellar toast for their reception, in addition to showing up on time for the rehearsal and the ceremony.
Joking aside, the role is easier today than it was when it began in Europe. In the 1600's, when marriageable women were in short supply in a local community , the grooms stole them from other communities with the help of the best man. During the ceremony, the best man stood next to the groom, hand on sword, ready to protect the couple from objecting family, friends and guests, and to keep the bride from being stolen back.
You can relax. No readiness for swordplay is now required!
If you are seating a mother or grandmother, be sure to offer her your arm by lifting your elbow in her direction with your wrist closer to you and your arm parallel to the ground. When she has taken your arm, bring your elbow down close to your body, letting her hand rest on your arm. Walk slowly down the aisle to her seat, and let her release you and sit down. Then return to where the groom is waiting for the ceremony to begin, or go to your place on the platform.
If you're carrying one or both of the rings, make sure you have them before entering the ceremony, and use your inside coat pocket to keep them safe until they're needed. If you have no pockets–because the wedding is really casual–wear the bride's ring on your little finger until the ring exchange takes place.
When toasts begin at the reception, have something prepared for your toast that will put people at ease and make them smile. It's okay to read your toast if it's brief. Make sure, either way, that you have practiced what you're going to say. You can find some good resources for the Best Man in Amazon books.
Good luck, Best Man, and have a great time!