A blockchain classically consists of a chain of blocks. This chain is usually called the mainchain. It links the individual blocks so that a blockchain is created. In addition, there are the so-called sidechains, which run next to the mainchain. They are, so to speak, branches of the mainchain. This means that they can exist relatively autonomously next to the mainchain.
For this, it is necessary that tokens are “transferred” from the mainchain to the sidechain. After all, it should not be possible to output these tokens twice. Instead, the tokens are “marked” so that they are (temporarily) no longer processed on the mainchain. The sidechain represents a black box. This means that many independent transactions can take place in this black box without the entire network being aware of it. Because this only reads the mainchain and does not see what happens in the background on the sidechain.