You finish a brake job, start the car, and the pedal feels soft or goes low - alarming after fresh pads. Usually it is a simple, expected thing; sometimes it means air. Here is what a spongy pedal after new pads means.
Pump the pedal first
The most common and harmless cause: when you compress the caliper pistons to fit new pads, the pistons are pushed back, so the first pedal press or two is long until the pistons move the pads back out to the rotor. Pump the brake pedal several times before driving - it should firm right up. Always do this after any pad change.
If pumping does not fix it: air
If the pedal stays soft after pumping, air likely got into the system - for example if you opened a bleeder, cracked a hose, or let the reservoir run low; see spongy pedal after bleeding and why the pedal pumps up. That calls for a proper bleed to remove the air.
Check the reservoir and for leaks
Make sure the reservoir is topped to MAX - compressing the pistons pushes fluid back up, so it may need adjusting; see overfilling brake fluid. And confirm you did not disturb a hose or fitting that is now weeping - see brake fluid leak signs.
Do not drive on a soft pedal
Never drive until the pedal is firm and high. Pump to seat the pads; if still soft, bleed the brakes and check for leaks - see spongy pedal causes. Then bed the new pads in. Use quality pads and fresh hardware.