A makeup trial is a trial makeup session with a potential makeup artist. Trials generally give the bride a sense for how the makeup artist would apply her makeup on her wedding day.
Some makeup artists are also hair stylists, allowing them to do a hair trial as well.
You can have as many makeup trials as you want to determine which artists are more suitable to your tastes.
The trials can be held at a salon, at the artist's home/studio or in your own home/hotels.