(an airplane) is experiencing turbulence
"Turbulent" air or water is not smooth. It's unpredictable and chaotic. When an airplane flies through turbulent wind, it shakes. To describe this situation, you can say that the airplane "is experiencing turbulence".
"Experience ___" means to go through an unusual situation. This word is also used in the expression "experience outages".
You can also use "experience turbulence" in the past tense like this:
A: How was your flight?
B: We experienced a little turbulence. It freaked me out!