Master Level Xp is what you need to 'finish every 11th step for almost all Master Level 'levels'.
Master Levels is a series of 'quests' (steps) that you can complete in order to unlock a new power. They are grouped in 10 levels, most of which have 11 steps. Step 1 through 10 of each Master level is usually killing something, and the final (11th) step of each Master Level is filling the Master Level XP bar. (That is the blue bar on the side of you /master popup).
x.1 through x.10 is killing stuff, and x.11 is the 'master level xp' step. Master Level 1 and Master Level 10 do not have MLxp requirements.
- Killing some named mobs. If you click on them they will show up as giving MLxp.
- Grouping with people that did not yet have the ML step done. But only if you have finished all the previous Master Level ranks up to the one you're at. (You don't get MLxp for grouping with someone who is doing ML8 when you're ML3 yourself, but you do get MLxp if you help someone get ML3 when you are ML8 yourself.)
- Killing enemies in RvR gives you MLxp as well.
Master Level does not 'stack' - you only gain MLxp for the step you are at, so once your MLxp bar is full, you must finish the ML before you can start filling the next MLxp bar.
You cannot finish MLx.10 before doing the .1 through .9 steps. (But this changed in the next patch, so you can do it then.)