Advantage numbers

I've seen this on numerous apps. When analyzing a game it gives a number in white's benefit, like (+1.87) or whatever. How is this determined? How reliable is it?

https://support.chess.com/customer/en/portal/articles/1444875-what-do-the-computer-evaluation-numbers-mean-like-2-25-

Regarding the reliability of these numbers:

Computers are getting better and better at chess. They are right almost all of the time, but they are not infallible. It's far more useful to use your own judgement and the judgement of Grandmasters where they disagree with the engines. If you disagree with the computer's evaluation, test it out. See if your analysis is better than the engine's! It's still possible in a few cases to prove that engine evaluations are wrong. 

Here's one example. Stockfish says that White is winning with a +8 advantage:

easy win bxa6!!   

Ya got me.

I shoulda said that en passant wasn't possible in that position.