Bitcoin's base layer is intentionally limited in its functionality and throughput in order to preserve highly valued attributes of the network such as independent verification and censorship resistance. In order to achieve better scalability, privacy, and versatility, developers build desired functionality as new layers of technology that anchor onto Bitcoin.
Covenants are restrictions on spending that allow for new functionality through assurances about how a UTXO may be spent. While Bitcoin has some limited covenants like timelocks, these are mostly proposed changes to Bitcoin.
Just kidding. You don't need to care about sidechains. Sidechains - such as liquid, rootstock, "defichain", sequentia, and other elements or mainstay projects are little more than centralized distractions at best and shitcoin scammers at worst. We aren't going to waste your time discussing them further.