You received a birthday gift from a coworker. You are opening it in front of her. It's a shirt for you. You want to make her think that you're happy about the gift, so you say this.
The expression "How lovely!" means that something is very lovely. "How ___" is a phrase to show that you feel a strong and surprising emotion about something:
How sweet of you!
Note that "How ___!" is only used by itself. You never use it in a sentence like "This shirt is how lovely."
The word "lovely" means "nice" or "beautiful". But it sounds more strongly positive than "nice".
Things that can be called "lovely" include:
- The weather.
What a lovely day!
- A person.
Don't you look lovely!
- Clothes, household decorations, and other pretty objects.
I found a lovely bedspread on sale at Macy's.
"Lovely" is often used by older and more conservative women.