Nominators secure the relay chain by selecting good validators and staking DOTs.
You may have an account with DOTs and want to earn fresh DOTs. You could do so as validator, which requires a node running 24/7. If you do not have such node or do not want to bother, you can still earn DOTs by nominating another validator.
By doing so, you become a nominator for the validator of your choice. Pick your validator carefully - if they do not behave properly, they will get slashed and you will lose DOTs as well.
While your DOTs are staked by nominating a validator, they are 'locked' (bonded). You can receive new DOTs in your account but you cannot stake as validator or transfer DOTs away from your account. You can un-nominate at any time to unlock your funds. Keep in mind that the un-nomination is effective in the next era.