Sidechains
Plain English
Separate blockchains connected to the main chain that can have different rules and features. Like building a smaller, faster road next to a highway. Assets can move between the main chain and sidechains via bridges.
Technical
Independent blockchains connected to parent chains through two-way pegging mechanisms. Sidechains can implement different consensus mechanisms, block times, and features while maintaining asset transfer capabilities with the main chain. They offer scaling and experimentation opportunities but typically trade some security guarantees for increased functionality or performance.