But = "mas". It is used in expressions that indicate opposition. Example: "Estudei muito, mas não consegui passar no teste". Plus = "mais". It is used in expressions that indicate intensity or sum. Example: "Vou comprar mais duas camisetas" ; "Preciso estudar mais"; "Ela escolheu a roupa mais cara da loja".

I think Rafael explained it very well, I just would like to add, that you probably confused these two words because they are pronounced in a very similar way. Also Brazilian native speakers say and write it wrongly, although the meanings are quite different. So, you should pay attention, even when Brazilian people talk to you, because they may be making this mistake.

You probably got confused because the translation is pronounced in a very similar way. "But"="mas", giving certain idea of opposition or restriction. On the other hand, "plus"="mais" (among other meanings, of course), giving an idea of addition.