Nominators secure the Blockchain by selecting good validators and staking DNAR.
You may have an account with DNAR and want to earn fresh DNAR. 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 DNAR by nominating another validator.
By doing so, you become a nominator for the validator(s) of your choice. Pick your validators carefully - if they do not behave properly, they will get slashed and you will lose DNAR as well.
While your DNAR are staked by nominating a validator, they are 'locked' (bonded). You can receive new DNAR in your account but you cannot stake as validator or transfer DOT away from your account. You can un-nominate at any time to stop staking your funds. Keep in mind that the un-nomination is effective in the next era, and that un-nominating does not automatically unbond your funds. There is an unbonding period of 28 days before bonded funds can be transferred.