You pop the hood and the brake fluid reservoir is below the MIN line. Before you just top it off, know that a low reservoir is telling you something specific - and there are really only two causes. Here is how to read it.
Cause 1: worn brake pads (normal)
This is the most common and least worrying cause. As brake pads wear down, the caliper pistons extend further to reach the rotor, and that extra piston travel pulls fluid out of the reservoir to fill the space. So a gradually dropping level often simply tracks pad wear - see when to replace brake pads. Fit new pads and the level rises back as the pistons are pushed in.
Cause 2: a leak (urgent)
The other cause is a fluid leak - at a caliper, wheel cylinder, hose, line, or the master cylinder. A leak is a safety problem because losing fluid means losing braking, often with a sinking pedal - see when a soft pedal is urgent. Look for wet spots at the wheels or under the car and check the fluid is not dropping fast.
How to tell which it is
Check your pad thickness. If the pads are worn and there are no wet leaks, the low level is just pad wear - normal. If the pads still have plenty of material, or the level drops quickly, or you see fluid anywhere, suspect a leak and get it inspected before driving.
Do not just keep topping it off
Never treat a low reservoir by repeatedly adding fluid without finding the cause - if it is a leak you are masking a dangerous fault, and if it is pad wear you should be replacing pads. Fix the root cause, then fit quality pads and fresh hardware.