Remarketing (also referred to as retargeting) is at its most simple the method of advertising on the internet to people who have already visited your business website. This can stimulate sales/conversions coming from customers who have visited your website but who haven't yet responded - the majority of people want to consider an offer at least several times before opting for it.
You can get more in-depth and specific than this (e.g. targeting prospects who have checked out particular products or parts of your website) - however the key idea of remarketing is fairly basic for sure.
As for how you do remarketing; there are a several different ways to do it. For most advertisers, kicking off on a self-service platform like Facebook Ads or Google Adwords is a good place to start off.
Both Facebook Ads and Google Ads feature a number of beneficial remarketing features, and make things relatively straightforward to start. If remarketing on Facebook, you'll need a Facebook Pixel installed on your domain. When using Google Ads, you can use a specific remarketing tag or just link your Google Ads account to Google Analytics and share the data that way.
If you're in the Business To Business sector, then you might also want to have a go with remarketing with LinkedIn also. You will need a LinkedIn Insight Tag to achieve this.
This short video outlines more about remarketing and how you can use it to grow your business?