The compressor face of a jet engine is one of the least stealthy parts of an airplane. Not only will the whirling blades, at some point, reflect radar energy directly back at whoever is looking for you, but the shape and rotational rate will identify you, because computers can count very, very fast.
Step 1 in dealing with this problem is to coat the inlet duct with radar absorbent material, because a lot of radar energy bounces off the duct wall several times on the way in and out again. High-level stealth, though, means physically blocking the line of sight with a "serpentine" duct (which is done on the F-22, JSF and Typhoon). But that can take up a lot of space, particularly with big engines, and isn't practical for a stealth retrofit or on some new designs.
The Super Hornet, for example, has short inlet ducts so line-of-sight blockage by curvature isn't practical. The solution was to install a blocker in the inlet duct - looking down the duct, you see what looks like a compressor face, but isn't. It's a fixed composite structure, RAM-coated. And of course any stray electrons that do make it through the blocker and hit the compressor have to make it out through the blocker again. Problem: what bends electron flux also bends airflow, so you can get losses.