Choose a spell of 9th level or lower that you can cast, that has a casting time, and that can target you or others (but does not cause damage). You cast that spell - called the contingent spell- as part of casting greater contingency, expending spell slots for both, but the contingent spell doesn't come into effect. Instead, it takes effect when a certain circumstance occurs. You describe that circumstance when you cast the two spells. For example, a contingency cast with teleport might stipulate that teleport comes into effect when you are in danger (half hit points or lower) and include your friends in the casting of the spell.
The contingent spell takes effect immediately after the circumstance is met for the first time, whether or not you want it to, and then contingency ends.
You can use only one contingency spell at a time. If you cast this spell again, the effect of another contingency spell on you ends. Also, contingency ends on you if its material component is ever not on your person.