To insure is to guarantee or protect the monetary value of something with, as you might imagine, insurance.
We insure ("protect") our homes and cars and other valuables against loss or damage from accidents by taking out insurance policies. Sometimes insure is used for other kinds of guarantees and assurances (like insuring that we will pay back our student loans, or insuring that our travel plans are in order before a vacation), but in those cases it's better to use ensure.