STP refers to Standard Temperature and Pressure, which are as follows:
Standard Temperature: 0 degrees Celsius or 273 K
Standard Pressure: 1 atm or 760 mmHg
If you use PV = nRT, you will find that universally, 1 mole of any gas at STP will have the same volume: 22.4L.
Problem: What is the volume (in liters) occupied by 49.8 g of HCl at STP?