I know I can't just stick two masks on the same clip but I recall a way around this so you can have the effect of two objects being masked - say a "color" mask for a blue object and a red object in the same scene maintaining color while the rest of the scene is B&W?
Duplicate your clip on top of itself and add a mask to that clip.
Repeat as many times as needed.
I will usually take the additional step to nest all the layers into a compound clip for the sake of keeping a tidy timeline.
(Remember to include enough handles inside your CC in case you need to slip or trim the shot later.)
I think he means that in the timeline, your clip has an in/out point. Making the in point earlier and the out point later would effectively give you "handles" or more media to extend later if needed. If you never use those "handles" then you are fine. But sometimes you might need a few more frames at the beginning or the end.