You tend to get what you pay for
Some companies offer differing levels of cover.
Most pay a pre agreed sum for a specific condition.
If you opt for lifetime cover then they pay that amount , for that condition, every year that you pay your premiums.
eg if your cat has an accident- hopefully a one off occurence- then they will pay up to the agreed amount for treatment. If you cat developes a condition eg diabetes then they will pay up to that amount every year that you are insured with them.
If you opt for the cheaper insurance option then you will find that you are only insured for that condition for a year and then they won't pay out anymore.
It really pays to read all the small print and assess what you think you might need.