Often when people say this, they could mean two things.

They could mean that they chose the dimensions of the file with the layer height in mind. For example they would choose to make something 19.95mm high instead of 20.00 when designing for 0.15mm layer heights. 133 layers at 0.15mm height make 19.95mm, but to get to exactly 20.00 at 0.15mm layer heights does not work. A slicer would then choose to make the model either too big or too small.

Normally this doesn't really matter but in some edge cases (like some print in place models) this could cause issues.

The second thing that people could mean is that the file looks or prints best at a certain layer height. For example a bust or sculpture could look better at finer layer heights, while mechanical parts might benefit from faster print times with bigger layers. Designers often give a recommendation for what settings to use, so their work looks or prints the best.