To come across is to stumble upon, to discover accidentally. Discover things rather than people, usually. Examples:
While going through some old photos I came across this one, which I thought might interest you.
I was surprised to come across this hint that he may have been lying.
There is another meaning of come across, probably colloquial AmE. In this usage, to come across means to give people an impression about oneself. Examples:
I don't trust him. He came across as slippery to me.
She comes across as diligent and helpful.