Why Every real component has the reactance it is supposed to have (nominal) plus an unintended stray reactance of the opposite kind (parasitic): an inductor has capacitance between its turns, and a capacitor has inductance in its leads and plates. At one particular frequency those two reactances are equal and cancel, which is the component's self-resonant frequency. Above that frequency the part behaves like the opposite type of component, so a choke can start acting capacitive and a bypass capacitor can start acting inductive.
Watch out Saying simply inductance and capacitance is close but incomplete, and it describes a deliberate tuned circuit; the point of self-resonance is that one of the two reactances is a parasitic that the designer never intended to put there.
Self-resonance = what you wanted plus what you got stuck with: nominal reactance versus parasitic reactance.