It's a play on an idiom. :)
To 'have a frog in your throat' means you're so nervous you can't speak... only croak! You usually get this feeling when the other person is very important, or is someone you're infatuated with.
So to overcome this feeling, you "swallow" whatever is in your throat - even if it is a "frog".
A similar idiom to "frog in your throat" is "cat got your tongue".
PS. Look for "Funny Little Frog" by Belle and Sebastian, it's a great song. ;)