A toxic link is a link which has probably a negative impact on your SEO. This can be, for example, a very unreliable online casino that makes a link towards your website which is a tourism website about Paris. First, this is not a reliable link and then this website has nothing to do with your website. There is no correlation between both activities.
According to Cocolyze, a toxic link is a link with a Spam Rating higher than 7.
Unfortunately, it is impossible to delete a link on a website which is not yours. You can try to contact the website but it is tough task.
Fortunately, Google created a solution: Disavow links. To disavow a link is just to tell Google "I don't want this link, I don't want you to take it into account".
In the "links" tab, we created a tool to help you to disavow a link easily.