From a grammatical perspective, venga is the present subjunctive form of the verb venir (to come) for the third person in the singular.

That means that it is used as a command - because this is one of the functions of the present subjunctive in Spanish - when speaking to someone whom you would address as Vd. in Spanish (¡[Que] venga [Vd.] a vernos mañana, señor! - Come and see us tomorrow, sir!) or when speaking about someone being referred to as él or ella (he or she) (¡[Que] venga tu amigo a vernos mañana! - Let your friend come and see us tomorrow!). As these examples given show, the conjunction que sometimes appears before such commands.

From this grammatical origin, the phrase venga has however taken on an idiomatic life of its own, semi-independent from its origin as a command and regardless of whether you address the person in question as tú (singular you, informal) or Vd. (singular you, polite). With this idiomatic function, the phrase becomes a kind of general exhortation, encouragement to do something (¡Ay, venga, vamos a ir a comer! - Hey, come on, we’re gonna go and eat! / ¡Hombre, venga, no te enfades! - Dude, come on, don’t get angry!).

Strictly speaking, if you use venga in this sense to speak to more than one person, you should pluralise it to vengan. However, because venga in this idiomatic sense has the function of a general exhortation etc., it has become something of a fixed expression, and so this rule about pluralisation is often not observed (¡Ay, chicos, venga[n], tenemos que limpiar la casa antes de que vuelvan! - Hey, boys, come on, we’ve gotta clean the house before they get back!).