There are various cash back websites that offer you cash when purchasing online, you simply log into your cash back account and search or browse the various merchant sites that are available and see what the cash back is. You then click the link to go directly to the merchant website and shop normally, but because you have clicked through to the merchant from the cash back site this is recorded and the cash back site is then rewarded by the merchant. The cash back site then either passes some or all of this reward received onto you.
If you use it for purchases you would make anyway, then you are saving yourself cash on something you would have bought anyway.
I use topcashback because they seem to pay the most cashback from all the cash back websites. They pay at least 100% of the cashback gained through merchants, and sometimes they even increase this to give you a bonus. Although obviously you can not get cash back from every merchant, I found that there is an extremely large database of merchants on topcashback and most of them offering cash back. I have managed to save quite a bit on online shopping by making sure I click through topcashback when purchasing online.