Delta = change and v = velocity

So delta v is change in velocity

Also known as acceleration. It's just the "spacey* term for it.

Now in space there's generally nothing slowing you down except gravity... So to add delta v you simply run your engine or burn your rocket. But that uses fuel which is why it's hard to "add enough delta v to overcome our orbital velocity"... You need a whole lot of fuel compared to the mass you're trying to move.