On the wikipedia page for supersingular isogenies, it says:
The supersingular isogeny Diffie-Hellman method works with the set of supersingular elliptic curves $E$ over $F_{p^2}$, where the number of points on any such curve will be $(p ± 1)^2$. An isogeny of an elliptic curve $E$ is a rational map from $E$ to another elliptic curve $E'$ such that the number of points on both curves is the same. For supersingular elliptic curves, isogenies are equivalently defined by points inside their kernel.
What does the bold statement mean?