This means if you do end up in a situation where you have a rental car and you have a flat tyre, this has happened while the car is on rent and is not covered under the standard breakdown assistance cover that is included in the rental, so a call out would be chargeable.
The best course of action is to use the spare wheel if the car has one, or use the puncture foam kit which will be provided should the car not have a spare wheel. You can then take to any tyre place and purchase a new tyre, however it MUST be the exact same make / model of tyre.
If you have purchased the optional, additional insurance to cover the tyres. You can contact the rental company and they will advise you as to which garage to take the car to, the tyre will be replaced and billed back to the rental company. If you do not have any additional insurance cover the cost to replace the tyre then it is the responsibility of the driver or the person named on the rental agreement.
If you are unsure you can always call and query with the supplier or refer to your rental agreement which will confirm if any additional cover was opted for at the rental desk.