1. "Mouth" (of a river)
This is the most common meaning of "aber". It refers to the place where a river meets the sea. Many Welsh place names include "Aber", like Aberystwyth, Aberdare, Aberconwy.
2. "But"
This meaning is less common, but it can be used as a conjunction, similar to the English "but".
3. "Upper"
This meaning is also less common, but it can be used to describe something located higher up, like "Aberllefenni" meaning "the upper marsh".
To understand the specific meaning of "aber" in a sentence, you need to consider the context.