The verb to be is very useful!
1. It can be used to mean 'to exist':
To be or not to be: that is the question. To live or not to live: that is the question.

As well as being an auxiliary verb it can express:
2.The progressive form, if it is followed by a present participle (-ing form) :
He is sleeping. He is asleep at the moment.
3.The passive voice, if it is followed by a past participle:
The table is set. Someone has set the table.
4. Obligation, duty, agreed actions (in the future), if it is followed by an infinitive:
The Prime Minister is to arrive this afternoon in Paris. The Prime Minister should arrive in Paris this afternoon.