The bottom piece of track – correctly called “track” because something, in this case the gooseneck, clasps around it – is, as others have said, a device for letting the gooseneck slide up and down for the purpose of adjusting the luff tension of the sail.
The upper piece of “track” is more correctly called the “cove” because the “slugs” on the luff of the sail slide internally in the cove.
The middle piece shown in your photo is a “parking place” for the slugs, which you feed in from above when you “bend on” (attach) the sail. The slugs sit one above the other in this piece of cove while the sail is in a “harbour stow”. This piece of cove therefore has to be permanently closed at the bottom, and it needs to be long enough to accommodate ALL the slugs. When the sail is “harbour stowed” (or “furled”) the “bunt”, (the “middle” of the sail, i.e. all the cloth) is let fall so it lies in folds alternatingly to one side and the other of the boom. The bunt is then held in place on the boom with “robands” aka “gaskets”. A roband is a piece of light line as long as it needs to be, but no more. You don't want loose ends (“Irish pennants”). The robands are tied around the furled sail at intervals of a coupla feet, and tied off with “roband knots”, a knot
you will know as a “square knot”.
Set up correctly, the gap between the top and the bottom cove (called “the gate”) is filled with a piece of cove mounted on a hinge so the gate can be closed, and the slugs can slide directtly from the “keeper cove” via the hinged gate cove up into the working cove on the mast. The reason you need the hinged gate is that having it permits you to have both hands free for the halyard. If you don't have the hinged gate, hoisting sail becomes endlessly frustrating.
and reefing are NOT the same thing. When reefing, the gate stays closed, and the slugs below the reef will find their own place in the cove. At the reef points there are gaskets permanently attached to the sail – or at least there should be. These gaskets are called “nettles”, and when you reef they are taken below the bolt rope in the foot of the sail and tied off there with the tension of each one adjusted to give you a nice “belly” in the sail. If the entire boltrope in the foot of the sail is fed into a cove on the boom, as it sometimes is, then, obviously, the nettles need to be tied off under the boom, but again you adjust their tension to give you a nice belly. The first step in reefing is to lift
the gooseneck a few inches so that after having completed the reef, the gooseneck can be brought down again by the gooseneck downhaul so as to put the proper amount of tension on the luff.
With all due respect to those who suggested above that the gap is there to facilitate reefing, you will see that that is not so. The gate is there ONLY to faciltate bending on the sail and striking it. It has nothing whatsoever to do with reefing or even furling. Set up correctly, it is doubtful that you would need to open the gate more than once a year or two.
There you go – more detail than you asked for, but all this stuff needs to be considered as a cohesive entity. If we can help in any other way, sing out :-)!