The ideal gas law says PV=nRT. If we assume that a gas is in a closed container where no gas is let in and none escapes, then we can calculate how pressure, temperature, and volume change over time.
If PV = nRT, then using algebra, we can solve for nR to get nR = PV/T.
In a closed container, the number of moles of the gas will not change. And R is a constant as well. So if P1, V1, T1, are the pressure, volume, and temperature at time 1, and P2, V2, and T2 are the pressure, volume, and temperature at time 2, then (since nR don't change P1V1/T1 = nR = P2V2/T2 or:
P1V1/T1 = P2V2/T2
Boyle's Law says P1V1 = P2V2 so, iyou can see that temperature has been removed from the equation. That's because they assume that temperature is held constant so does not play a part in this particular situation.
So, the amount of moles must be constant (the gas must be in a sealed container where nothing escapes and nothing comes in) and temperature must be constant.