Immobile ions are not able to move or fixed of ions e.i positive or negative ions.

In P-N junction, P region holes are charge carriers while the N region electrons are the mostly charge carriers. During the formation of p-n junction, concentration density gradient of the charge carrier changes. Therefore, holes diffuse from P region to N region while electrons diffuse from N region to P region.  As a result electrons, it leaves behind an immobile positive ion or donor ion on N side and develops positive charge layer. Similarly, when a hole diffuses from P region to N region, it leaves behind an immobile negative ion or acceptor ion on P side. As a result the hole develops negative charge layers on the p-side of the junction.