A first look is a private moment arranged before the ceremony, usually somewhere quiet, usually with only the photographers nearby. Couples ask us whether they should do one as though there were a correct answer. There is not.
What it buys you
Time, mostly. Portraits made before the ceremony mean the hour afterwards belongs to you and your guests rather than to a list of group shots. It also takes the edge off: a lot of people are calmer for the rest of the day once the first moment is behind them.
The ceremony reads differently when the nerves are already spent.
What it costs is the surprise of the aisle. If that moment is the one you have been imagining, keep it — we will build the timeline the other way around and nothing is lost.