The plastic tank on top of the master cylinder is the brake fluid reservoir. It is simple but important - it is where you check and add fluid. Here is what the master cylinder reservoir does.
It stores and feeds brake fluid
The reservoir holds a supply of brake fluid and feeds it to the master cylinder, which pushes it through the system when you brake; see what a master cylinder does. It keeps the system topped so no air gets drawn in.
Why the level drops
As brake pads wear, the caliper pistons extend and hold more fluid, so the reservoir level slowly falls - that is normal. A sudden or low drop, though, points to a leak; see low reservoir. Do not just top off worn-pad-low fluid without checking; see reservoir overflow.
The MIN and MAX lines
The reservoir is marked MIN and MAX. Keep the fluid between them - too low risks air in the system, too full can overflow when you push caliper pistons back during a brake job.
Care tips
Keep the cap sealed (brake fluid absorbs moisture), use the correct DOT fluid, and never let the level fall to empty. Watch the level as a clue to pad wear and leaks. Keep the brakes fresh with quality pads and hardware.