Unbonding is an on-chain parameterized period of time upon which all the delegators must wait for their tokens to return fully available.
It starts when you remove from the [staking] status your tokens and until the unbonding period has passed, the tokens are locked.
The unbonding period lasts 21 days on Cosmos Hub, ensuring a variety of security measures in the specific network.
This number might vary from chain to chain (Osmosis=14, Sentinel=28,...)
Within the unbonding period, you will not be able to do any type of operation with your tokens and you will not receive staking rewards.
You do not need to unbond your token to change the validator: instead, choose the [re-delegation] option available in the [Staking] section of Keplr Dashboard.