To ensure that something will be done means to make it certain to happen.
“When amending the contract, the lawyer ensured that her client received a better deal.”
To insure someone or something means to protect the person or object against risk by regularly paying an insurance company a sum of money. In the event the person is injured or killed or the object is damaged or lost, the insurance company will then pay out a sum of money to cover the injury or loss.
“The company was insured against accidents in the workplace.”