“Jolly Good”

What does this phrase mean?
Nigel’s students, in particular, are rather confused by it. Last week, when someone passed her FCE exam, Nigel said “jolly good!”
When another student said he was enjoying the course Nigel again said “jolly good”.
Yesterday a rather difficult student told Nigel he was leaving the school and Nigel’s reply was, once again,” jolly good”.
Trying to clarify this language point, several students consulted both “Red Murphy” and “Blue Murphy”, grammar books but with no success. Finally Nigel’s students asked him if he could build in a lesson on “jolly good” in the programme for next week. Nigel’s reply was – you’ve guessed it! – “jolly good!”