SER gets conjugated into SEJA in two very common, but different situations.
SER in the Imperative
The imperative is usually a command. In these instances seja is used to express “be!” or, “will be”.
~ be: Seja bem vindo! ➜ be welcome!
~be: Seja feliz! ➜ be happy!
SER in the Subjunctive
This is the present subjunctive form of SER. It translates as,”is” — but with a sense of doubt.
~ will be: Qual quer seja. ➜ whatever it is
~ rather: Ou seja... ➜ or rather...