SRP is short for "Stat Raising Points". SRP are used to up your monsters' stats. Instead of gaining boring old experience, your monsters get SRP. Below is the formula for how SRP is figured out.
[level + (stat you're raising's current value) + (work level / 2)]
So if your monster's level is 3 and their stamina is 9, it will cost 12 SRP to raise that stat one.
Every time you raise a stat, your monster's level increases by one.
When you go out for the day, your monsters are souped up to kill. After a while, they get tired. Under valor, you gain a bit more SRP than usually from mob kills. Valor is reset every night.