Informal use meaning ‘also; in addition’.
Tickets are hard to find, plus they’re expensive.
Les billets sont difficiles à trouver, de plus ils sont chers.
Synonyms
in addition
on top of that
what's more
as well
Colloquial mathematical use: ‘to add’; requires verbal paradigm.
If you plus 4 and 7, you get 11.
Si vous additionnez 4 et 7, vous obtenez 11.