How to delegate and earn rewards?
Ada that is held on Cardano represents its holder’s stake in the protocol. The size of the stake is proportional to the amount of ada held. Users who hold a stake in Cardano can earn passive rewards for validating blocks.
Since not everyone has the time, knowledge, or resources to run a stake pool, ada holders can delegate their stake to a preferred pool and let an operator maintain their stake on their behalf. This allows everyone to participate in the consensus and earn rewards without having to continuously run the node online. The higher is the stake in a pool, the more rewards will be assigned to its owners.
Note that you can spend your ada at any time, regardless of how you delegated it.
Ada holders can delegate their stake using Daedalus or Yoroi wallets. Here are some useful articles to review: