What are Self Sealing Tyres?

Self-repairing tyre (Self sealing) 

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

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

Example of self sealing by Continental