Use this phrase to politely offer to do something.
For example, you can say this to someone who's visiting your company from another country:
I'd be happy to show you around, if you'd like.
You can also offer to do something with "I'll be happy to ___". This version is more appropriate after "if":
If this one doesn't work for you, I'll be happy to bring you a new one.
These phrases are polite, so you can use them with people you don't know very well.