"Rather" is an adverb which is similar in meaning to "pretty ___". You use "rather" before an adjective to show how much of that quality something has:
It's rather hot in here, don't you think?
"Rather" is stronger than "a little" but not as strong as "very". It's a more formal-sounding word than "pretty" or "kind of". It also has a little bit of a negative sound. People often use it when complaining (in a polite way) about something:
Her cooking was always rather salty.
If you use "rather" to say something nice, it carries a feeling of surprise:
The performances were rather good.
There's another meaning of the word "rather", which is used when choosing one thing instead of another:
I'd rather eat out tonight.
It's probably best to think of these two uses of "rather" as totally different words.