Understanding the delegation mechanism
As Cardano is a proof-of-stake system, holding stake, or owning ada, means that you can buy goods or services with your purchase and payment power. In addition, you also have the right and obligation to participate in the protocol and create blocks.
These two uses can be separated by the delegation mechanism, meaning someone who owns ada can keep the spending power, while delegating the power to participate in the protocol to someone else, a stake pool. It is important to note that funds can be spent normally at any time, regardless of how they are delegated.
What is a delegation certificate?
A delegation certificate is a cryptographically-signed piece of metadata in the blockchain that transfers staking rights from one staking key to another.
A delegation certificate is published on the blockchain, therefore, transaction fees apply. If there are conflicting certificates, as in you delegate to someone and then to somebody else, two delegation certificates will exist. There is a rule to break the tie, and this is whichever is later in the blockchain wins.