Assure, ensure and insure all mean “to make secure or certain.” However, there are some distinctions in usage.
“Assure” refers to people. To assure people of something is to make them confident of it. “Bob assured her he would be on time for the opera.”
“Insure” today primarily refers to insurance. “The policy insures you against losses from fire and theft.”
“Ensure” means to guarantee. “We try to ensure the accuracy of our reporting.”