To ensure means to make certain that something happens or to guarantee a particular outcome. When you ensure your backpack is packed the night before school, you're making sure you won't forget anything important. When a chef ensures the food is cooked properly, she's taking steps to guarantee it's safe to eat.

The word suggests active effort and responsibility. You don't just hope something works out: you take specific actions to make it happen. A teacher might ensure every student understands a lesson by asking questions and offering extra help. A pilot ensures passenger safety by completing a thorough pre-flight checklist.

People sometimes confuse ensure with insure, which specifically means to protect something with insurance, like insuring your house against fire damage. When you want to express making something certain, ensure is the right choice.