As well as does not have a ‘precise’ meaning: the meaning varies in different registers.
In formal registers, it means approximately the same thing as “in addition to”: it heads a supplemental phrase and marks its oblique as another entity which also participates in the role of the entity to which it is attached. Usually the oblique is ‘old information’—an entity previously introduced into the discourse.
X: We’re discussing B.
Y: I beg your pardon, we’re discussing A as well as B.
Note that in this use the as well as phrase can move:
As well as B, we’re
In casual speech and writing, however, as well as is often used as a conjunctive phrase equivalent to and.
A as well as B were present.
This use in my experience is motivated by an effort to avoid repeated ands in a discourse, a sort of ‘elegant variation’. It doesn’t really add any nuance to the discourse, and it makes the syntax harder to parse for those who expect as well as to introduce a supplement; so it should probably be avoided.
Note, by the way, that with either use you can make things easier on your readers/hearers by maintaining parallelism between the adduced or joined entities if as well as appears in the conjunction position:
I want to go skiing
as well as ⇕
have a beach holiday.
I want to return the books to the library
as well as ⇕
fetch the children from school.