What does 2.5 um mean in terms of measurement?

Answer 1

I assume you meant 2.5 μm, but couldn't put the μ character into your question. If so, a μm, micrometer, or micron is a unit of length equal to 10-6 meters. 2.5 μm is therefore equal to 2.5 X 10-6 m, which is about equal to the width of a single strand of spiderweb silk.