Excess G Forces

It means that the forces acting upon you are too great and dangerous. To stop it happening dont climb or deascend too fast 😉

Just to add...

The main cause of excess G force isn't necessarily climbing or descending too fast, but pitching up or down too fast which is what PW probably meant. You can ascend at around 3000fpm and have relatively little G force present.

If you have a joystick, make small movements and don't move it frantically, especially in smaller aircraft 😉