Stansted Airport is one of the more expensive airports when it comes to picking up or dropping off using a car. Fortunately the train station runs very close along with the bus terminal, so these would be your first options if possible.
If you do have to use a vehicle, currently the cheapest option for pickup/drop off is the Blue Zone car park. This offers 30 minutes free parking, but it is a good 6-8 minute walk from the terminal, with the last 100 metres going up a ramp going to the terminal (downhill on the way out!). To get there turn left out the doors past the cab office and just keep walking straight. You will be under a covered walkway for a period but the last few hundred metres is 'out in the open', so keep this in mind if it is raining. There is a video here to show you the way.
If you don't mind forking out a bit and want a meet & greet type service you can use the Orange Zone car park. This is a lot closer, basically 3-4 minutes up some walkways and ramps. It will cost £10 for 30 minutes, and £15 for an hour, so gives you a bit of time to play with.
The most expensive place to park is on the platform outside the front of the terminal, you go up the ramp in the car and no ticketing so you don't know you are being charged until you get to the other end. This area will cost you £7 for 15 minutes, and £25 if you go over 15 minutes!
This is how Stansted Airport is at the moment, things will probably change again after Covid-19 and return to where it was previously. The free pickup drop off area will be back to the 'Mid Stay' car park which requires using the free transit bus which runs every 5-10 minutes.
Airport Cabs UK prefer to pick up and drop off using the Blue Zone and will provide more detailed instructions on accessing the area.
See below for video to Blue Zone car park.
Stansted Airport Meeting Point
Stansted Airport Information