Necessary condition $A$ means that $A$ that must exists in order for $B$ to occur, however, $A$ alone doesn't guarantee that $B$ happens.
Sufficient condition $A^\prime$ means that if $A^\prime$ happened then $B$ will inevitably occur.
Getting good grades is a necessary condition to get accepted into a prestigious university
Solving the Riemann hypothesis is sufficient to get accepted into a prestigious university.
So in short, a sufficient condition guarantees the occurrence of another condition
But a necessary condition doesn't guarantee it .
Also assume that $S$ is a sufficient condition for $B$
and $N$ is a necessary condition for $B$.
It is worth mentioning that if $S$ occurred it includes as well that $B$ occurred.
Sufficiency is a stronger notion.
If someone solved Riemann hypothesis,It is very obvious that he have
extremely good grades , but it is not true that every one who gets
good grades will solve the Riemann hypothesis
The necessary condition is automatically met when the sufficient condition is met.
If the necessary condition is not met, the sufficient condition is automatically not met.