In the blockchain ecosystem, testnets are considered to be entirely separate networks that are used for testing purposes. As such, testnet tokens allow developers to build and test their applications all without risking any monetary value.
In the Tezos ecosystem, Mainnet is the main Tezos network, however, Mainnet is not recommended for testing.
In order to conduct testing on the Tezos blockchain, there is one test network available for the current protocol, and one test network for the protocol which is being proposed for voting. The former is very important as users need to test their development with the current protocol. The latter is also needed to test the proposed protocol and its new features, both to decide whether to vote yes and to prepare for its activation.
After the intended protocol of a test network is activated (such as Edo for Edo2net), the protocol no longer changes because this could break the workflow of some users while they are testing their development, as they may not be ready for the new protocol. So every time a new protocol is proposed on Mainnet, a new test network is spawned. This also makes synchronization much faster than with a long-lived network.
Here are useful resources for testnets on Tezos: