An insurance excess is defined as "the amount payable by you in the event of a loss" and is the part of a claim you as the insured would have to pay out yourself (in most cases "deductible" from the claim amount paid out). This is essentially the uninsured portion of your loss as you would be left to cover this cost.
So when it comes to renting a vehicle, the excess is the amount you are liable to pay the car hire company towards any repair costs should the rental vehicle suffer damages, similar to that of a standard personal car insurance policy. The car hire providers damage / theft insurance cover will cover the rest of the costs should there be any as long as all the rental terms and conditions have been met.
The excess amount varies between car hire providers but normally ranges somewhere between £200 and £2,000 and can be far more than the original rental cost.
Sometimes the cost of repairs are not as high as the insurance excess, in these instances it willl be just the cost of the repairs that are payable instead of the full excess.