STP stands for Standard Temperature and Pressure.
It is for 0°C or 273°K, which are the same value. That was the temperature.
The pressure is actually 1atm, which is the amount of pressure the earth's atmosphere puts on us at sea level, it is also equal to 101,325 pascals, both of which are the same value.
Its good to have them standard, because its easier to compare and contrast two gases. If they are at different pressures or temperatures, they have different volumes, and it makes it really hard to compare the nature of the gases.