A Free for all website (or FFA website) is a website where you can add an advertisement for your website solely for the purpose of getting a backlink. Free For All websites are a poor black hat marketing technique and do not contribute anything to your site for SEO.
SEO is about content and links. In the olden days of SEO when spammers were running amok any links were of value. FFA sites, also known appropriately as link farms, popped up with the opportunity for everyone and anyone to post an add with a backlink and anchor text of their choice to their website.
Now, however, FFAs provide absolutely no value and any link juice that's passed on will be poisoned and likely to give negative signals to the search engines about your website.
It's easy to see why people thought link farms were once a good idea, but not as easy to figure out the benefit to the owner of the link farm. Often the advertisers were required to input there details, such as an email address, which would be sold on to spammers.
Link farms, Free for alls, FFAs, or whatever you want to call them are one of the worst types of spam and you should steer well clear of them.
It's often the case that an SEO will walk into a situation where the previous digital marketing efforts were pretty much spam-based – nightmare! One of the easiest to spot is spam links from low quality websites – any decent quality link analysis tool will be able to give you a rundown.
The first thing to do is attempt to contact the owner of the FFA and ask them to remove the link to your website. If this doesn't work you can disavow the dodgy link using Google Search Console's and Bing Webmaster Tools' disavow links option.