I have been coming across the "set seed" command while generating random numbers and while reporting bootstrap standard errors (example: set seed 1073741823) and "seed" while doing non-parametric regression (example: npregress kernel citations fines, reps(200) seed(12))
Can someone please explain-
1. What does "seed" and "setting seed" exactly mean here? What is it's significance?
2. Why do we have to set it?
3. What happens when we don't set seed and don't use "seed(12)" in the npregress command?

Thanks in advance!