what is the meaning for Ambient?

Answer 1

Ambient does mean "Surrounding" or "Encircling", but in chemistry it also implies "it does not matter what this is".

A chemist uses the word when something may not have been measured or does not make any difference for the sake of the reaction being studied.

Ambient temperature means any temperature typical of room conditions.
Ambient pressure means any pressure normally encountered in the atmosphere (usually at lower altitudes).
Ambient light might mean normals levels in a typical laboratory.

Answer 2

The conditions that surround the situation.