Google ad Remarketing is an online advertising that allow the site to ads to the user that have already visited to your site.
Remarketing is also know as retargeting, and can help to increase your conversion rate and also increase your ROI, this is because it is more likely for a customer who are past visitor to your site are more likely to complete the task from your site.
In order to do Remarketing what you can do you just have to add a piece of code which is called tag or pixel and through this visitor will be added to your Remarketing audience through browser cookies.
No doubt this will increase your cost for marketing, as you are targeting more narrow audience which is more specific.
So what is the need for Remarketing, it is because this allow you to be connected with the audience even after they have left your site.
Also by Remarketing it is seen that they rate for clicking the ads are increases and thus the conversion rate.
But there is also one down point to this which is the customer has option to change their setting from Advhoice program and can opt out of seeing the retargeting ads.