The Japanese copulas da (plain speech) and desu (polite speech) are the English equivalent of am, is, or are.
1. The A is B construction
Yamada-san wa bengoshi da/desu.
Mr Yamada is a lawyer.
2. As a substitute for other verbs (when the meaning is clear from the context)
Watashi wa omuretsu da/desu.
I'll have an omelet.