PPS is a name which is usually given to the output of a GPS module. PPS stands for "pulse per second."
Now I will talk about the PPS signal which comes from a GPS module. Because this pulse comes ultimately from the GPS system which has very accurate timekeeping, you can be very sure that it gives an accurate time stamp which will not drift in the long term.
Now let's talk about clocks used in microprocessors. A typical crystal oscillator may have a tolerance of 50 parts per million (50 ppm). Let's say it is a 1 MHz oscillator. If you set a timer to zero, and count clock pulses, you don't know if the true frequency of the clock is 1,000,000 Hz or 1,000,050 Hz or 999,950 Hz, because of the 50 ppm tolerance. So if you count out 1 million seconds worth of time (1 billion clock pulses) you may be off by as much as 50 seconds. One million seconds is about 11 days.
However, with PPS, assuming it is locked to the GPS network, after 1 million seconds, you will count exactly 1 million pulses, because the 1pps signal is extremely accurate (based on the whole multi-billion dollar GPS network).
It is also possible to basically calibrate the local oscillator against the PPS signal so that you can correct for the 50 ppm tolerance and obtain a clock that is both fast and accurate. But I think that gets a bit complicated. I am sure people have worked out all the details, and you can search for them if you are interested.