What is a Roche Lobe? When dealing with Binary systems and their evolution and mass transfers in the systems, why are equipotential surfaces a point of interest? 
The one which is called the Roche lobe. Why is the effective potential in these rotating frames a property of interest? Why masses inside these equipotential surfaces should be gravitationally bounded (and what it means for them to be gravitationally bounded to a body) ? What happens if objects go beyond these equipotential surfaces? 
So many questions better if you can answer them all answering 'What's Roche Lobe ?'
 A: Here's the idea:
Let's forget about astrophysics for a second.  I've got a stationary bucket of water.  What shape does the surface take?  Of course, it'll just be a flat plane (ignoring surface tension and capillary action effects).  You might say that this is boring, but then, I'll point out that this is a constant potential surface, according to the potential $V = mgh$.  If I add water to the bucket, I just pile more on top, but the surface will still settle down to a flat surface at a constant $V$.
Now, grab the bucket with your hand, and spin around in a circle.  The surface of the water will now be curved.  How will it be curved?  So that it has constant effective potential.
OK.  Let's go back to talk about a binary star system.  If I'm close enough to one of the stars, a surface of constant potential is just kind of a deformed ball around one star.  So, if I dump a bunch of gas in that star, it will just settle down on this surface of constant potential, and be fine.  As I dump more and more gas, the surface becomes one at a higher and higher potential, and gets farther and farther from the star I'm dumping on, which makes it closer to the other star, which distorts the surface of constant potential more and more.
Eventually, the surface of constant potential doesn't belong to one star, but rather, to both.  At this point, if I add gas, it's free to go from the one star to the other wihtout losing potential energy.  But, I haven't been adding gas to the other star, so, when it leaves the first star, there's nothing over there holding it up, so it's free to fall into the second star.  So, it accretes over.
And this is all the Roche lobe is -- the smallest potential equipotential surface that goes around both stars.  And the key point is that if you load up enough gas on one star that the top layer hits the Roche lobe, it will start accreting matter to the other star, because it can no longer localize the gas to just its orbit.
