What's a Self sealing tires?

Self-repairing tire (Self sealing) 

Several brands, such as Michelin (Self Seal), Pirelli (Seal Inside) and Continental (ContiSeal), offer an alternative to run-flat tires which is the self-repairing tire. These have a substance inside the tire that seals the hole where air escapes. This has the advantage of keeping a tire flexible and does not necessarily require a pressure monitoring system. It can also be mounted on all vehicles. No need to change the tire immediately, as long as the tire does not lose pressure, it does not need to be changed.It is only recommended to have the tire inspected by a tire specialist if we realise that an object has penetrated the tire

This technology is not immune to flat due to a puncture in the sidewall of the tire. The self-repairing substance is only applied to the tread. 

Example of self sealing by Continental