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.
Active vs. Inactive Nomination
When you go to the Account actions under staking page, you should see your bonded accounts and nomination status. If not, you can follow this guide to configure it first. Your nominations will be effective in the next era, that means roughly 6 hours on Kusama and 24 hours on Polkadot.
Suppose you have nominated five validator candidates, but three out of five were elected to be the active validators, then you should see two of your nominations as inactive. Active nomination means your nominated validators have been elected to be in the active validator set, whereas inactive means they did not get elected.
Possible effect of inactive nominations on rewards
As long as you have nominated more than one validator candidate and at least one of them got elected, your bonded stake will be fully distributed to the elected validators. That being said, you still will have the chance to get 0 rewards if you nominated very few validator candidates and no one got elected. So it would be safe to choose as many trustworthy validators as you can (up to 16) to reduce the risk of none of your nominated validators being elected.
What to take into consideration during nominating
As a nominator, if you only want to know the profit each validator made for each era, you can go to the Returns section under the staking page by inputting the number of tokens you would like to stake to check it. Then, nominate those who have a higher profit. However, that does not guarantee the right way to evaluate the validators' overall performance.
One example would be if a validator is regularly offline, their nominators most likely would get fewer rewards than others. More importantly, when many validators are unreachable, those nominators who staked with them will be slashed.
Thus, to be a smart nominator, it would be better to look at their histories such as blocks produced, rewards and slashes, and identity (if they have it set). Moreover, a nominator should do comprehensive research on their validator candidates - they should go over the validators' websites to see who they are, what kind of infrastructure setup they are using, reputation, the vision behind the validator, and more.
These concepts have been further explained in the Why Nominate on Polkadot & Kusama video and What to Consider when Nominating Validators on Polkadot and Kusama and Nominating/Staking on Polkadot and Kusama