Cryptocurrencies are said to be burnt when they are permanently removed from the supply chain with an intended purpose. The burning of cryptocurrency is somewhat similar to losing your assets in an accident or similar event where you end up with a depreciating value.
The token burning is mainly operated by the development team on a cryptocurrency/asset class by sending them to an eater address. This restricts the access of the cryptocurrency in the Blockchain, although the balance remains publicly available.
The burning of cryptocurrency involves the project developers' team buying the tokens back from the market. The most common goal behind burning tokens is to decrease the supply while increasing the assets price. This phenomenon often grabs the attention of investors and traders who were looking for just the right opportunity.