Normally a low brake fluid level just tracks pad wear. But if your fluid is low and the pads still have plenty of material, that rules out the usual explanation - and points to something you should check. Here is what it means.
Why fluid normally drops
As pads wear, the caliper pistons extend and pull fluid out of the reservoir to fill the gap, so the level slowly falls - see why is my reservoir low. That is normal and harmless, and topping up (or fitting new pads) restores it. The key word is that it tracks WORN pads.
Low fluid + good pads = suspect a leak
If the pads are barely worn but the fluid is low, the pad-wear explanation does not apply - which means fluid is likely escaping. Check for a leak at the calipers, wheel cylinders, hoses, lines, and master cylinder; see brake fluid leak signs and leaking caliper. A leak is a safety issue.
Or a hidden internal loss
A master cylinder can also leak internally (past its seals) without a visible drip, slowly lowering the level while giving a sinking pedal - see when a soft pedal is urgent. And spilled fluid from a recent service can make it look low.
What to do
Do not just keep topping it off - find why it dropped. Inspect for wet spots at every wheel and under the car, and check the pedal feel. Fix any leak before driving, then keep the system healthy with quality pads and fresh hardware.