A term spawned from two previously immature and overused insults 'your mum' and 'your face'. Although these two retorts were originally used by the illiterate and mono-sylavic lesser-beings of british culture the combining of the two has only recently been adopted by somewhat higher brow citizens.
Although used gramatically in much the same fashion as both its parent phrases the use of 'Your mum's face' is generally accepted to be a satirical take on modern chav culture.
"Man this seat is comfy"
"your mum's face is comfy"
"Don't you think this ice cream is delicious"
"Your mum's face was delicious last night"
adj. "your mum's baps" signifies a thing or a situation that is generally very good (in essence like your best mate's MILF-of a-mum who, despite being twice your age, has the most luscious breasts in the village).
A phrase used to put pressure on the person who is about to focus or carry out an important action, in 90% of cases resulting in the person performing an epic fail.
The term is so useful as there is not a single person on the planet that will not get intimidated by a mother's look of disapproval and even a thought about your mother watching you fail makes you nervous as hell, so you will almost certainly cock up whatever important thing you are doing.
Jack: *tries to take a penalty shot*
Troll: Your mum's watching!
Jack: *feels nervous and pressurised, then realises that mum is not really watching* Fuck Yeah.