A trimmed surface is a surface that has a portion removed by an embedded curve. The curve will divide the surface into two pieces. For example a square surface with a circular trimming loop will form either a square with a hole in it or a disk depending on which portion of the surface you want to keep.
This orange surface is the top portion of a soap bottle mold. It must be trimmed by the red curve to reflect the correct shape.
This blue surface shows the result of trimming the orange surface with the red curve.
See trimming restrictions for the rules for trimming surfaces.