‘Be’ is a verb because it equates a noun to doing or being another noun or adjective.
‘To be’ is a central verb in English and is used not just as a lexical verb (on its own – ‘I am human’ where ‘am’ is a conjugation of ‘to be’) but has auxiliary functions as well.
‘You are going’
‘You’ is a pronoun.
‘Are’ is a conjugation of ‘to be’ and is the auxiliary or helping verb in this situation.
‘Going’ is in the gerund form because it ends in ‘-ing’ and is the lexical verb in this case because it actually describes what is happening while ‘are’ is merely grammatical.
Some languages don’t have a word specifically for ‘to be’ and some philosophers regard it as a linguistic abomination because it implies assumptions about existence that aren’t necessarily true.