I have 20 amp breaker to barn which they run my lights, now I would like to buy a 90amps welder how can I wire this up? Do I have to run mor wire

Yes, the welder needs a circuit capable of handling that load. I doubt the 20 amp circuit can handle the welder.

What does 90A mean? Is that the output?

If so tell us what the input amperage is. The output amperage is meaningless as far as wiring it goes.

Also, how far is the barn? If it is quite far then larger wire will definitely be needed.