Turkey bought several S-400 anti-aircraft systems from Russia. They will be delivered soon. But the USA threatens sanctions and economic attacks.
Turkey has some number of United States made planes. The natural way to set up the S-400 anti-aircraft system is by flying planes above it and targeting them with the system (without firing, just aiming). The people who will be doing this targeting will be from Russia. They will then train Turkish personnel to do it.
See the problem? The US doesn't want Russia to be able to practice targeting their planes with their anti-aircraft system. In fact, the US regards this as a violation of the agreements made when they sold the planes to Turkey. The idea was that the US would train Turkish pilots so that they would be able to use the planes. But now, the Turkish pilots will be training Russian personnel to target the planes when the planes are following US practices.