A Safe Jump is a meaty jump-in that is commonly used to beat reversal attacks like Sol's Volcanic Viper or Ragna's Inferno Divider. Safe Jumps take advantage of the fact that you can cancel the recovery of a jumping attack by landing and immediately block afterwards. This is how safe jumps get their name: these jump-ins are safe against reversals.
A safe jump will naturally block reversals, thus allowing you to not worry about them while doing Okizeme.
Safe jumps are sensitive to timing; do it too early and your attack will miss the opponent completely, do it too late and the opponent can successfully hit you out of your attack. Naturally, that means faster reversals like Sol's Volcanic Viper and Chipp's Beta Blade are harder to safe jump compared to slower ones like Ky's Vapor Thrust and Axl's Artemis Hunter. Certain safe jump setups won't work against faster reversals at all due to the safe jump not having enough time to land before the reversal becomes active.
Moves that are active on frame 1 are also a way to defeat safe jumps since there's no time for you to land and block/evade. Examples of this include Blitz Shield in Guilty Gear XRD and Hakumen's Yukikaze (the counter super) in BlazBlue.