A sidechain is an alternative chain of blocks to the Bitcoin blockchain and constitutes an external solution of scalability.
This chain contains tokens that represent a certain amount of bitcoins: when you want to use it, you sends a certain quantity of BTC to a smart contract that will keep them locked and allow you to carry out some transactions with these tokens. If the user wishes to exit the sidechain he will be able to get his bitcoins again by returning the tokens to the address specified by the smart contract.
Bitcoin's most famous sidechain is currently Blockstream's Liquid Network.
The sidechain solutions are not comparable to the Bitcoin blockchain from a decentralization the point of view. These networks are in the hands of federations, in which the number of nodes is limited in order to guarantee a higher speed of transaction execution.
When considering the use of sidechain it is mainly done to take advantage of features not present in the Bitcoin blockchain or to have faster and more privacy-oriented transactions. However, trust comes into play: we must trust that the various members of the federation retain a reserve of 100% of the assets represented by their tokens and that these nodes are in sufficient numbers not to be easily attacked from the outside, guaranteeing that the network is not disabled.