MWD stands for "measurement while drilling." That means while oil or gas drilling is going on, people are using special tools to measure what's happening down in the hole where the drilling is happening.
Think of it like a doctor using a special tool to look inside your body and see what's going on, like if they were looking at your heart or your lungs.
With MWD, people can measure things like the temperature, the pressure, the direction the drilling is going, and the type of rock or dirt they're drilling through. This information is really important for the people who are doing the drilling, because it helps them know if they're in the right spot for finding oil or gas.
So, MWD is like a special tool for measuring what's happening deep down in the ground, while drilling is happening.