Computer Game Analysis Question

Good evening.  I have a question regarding games being analyzed by computers (Fritz in particular but I know others do this as well). 

When they give an alternate variation, I know that means it's better than the move I made.  What I'm not sure about is if it is only better if the whole variation were played, or is the first move of the variation the best regardless of my oponents next move.

As an example, say I moved 15...Qb3 and computer analysis gives a variation of (15...exd5 16. 0-0 Ne4 17. Rfe8) So in this example, is exd5 only better than the move I played (Qb3) if the whole variation were played or is it the best move at that time regardless of what comes next?

Thanks in advance for any help, it's much appreciated.

The whole variation shows what the computer currently believes is best. The end of the variation is likely to change, though, since it represents the farthest reaches of its current analysis. Hope that helped!