Meaningless noise that can be used to mean anything, depending on body language and vocal tone.
A common use is as a greeting. For this, the body language needs to be firendly - smiling, raised arm etc - and the word needs to be said loudly and clearly. It is particularly effective if the word is held on (in other words, saying merrrrrrrrrrrrrr).
Another common use is just to drop it in moments that lack conversation, or to express boredom. Again, it works best to hold the word (merrrrrrrrrrrrrr), and to say it only through one half of your mouth as if muttering.
It can be used as a way of agreement. In these cases it should be short, clear and often preceeded by 'ah'.
The word comes from Buckinghamshire, UK and is the nonsesical muttering of a certain someone.
What the merr