A disavow is a backlink you don not want Google to take into account toward your backlink profile. Low ranking websites that link back to your website only hurt your overall domain rank within Google. If you have low ranking backlinks, potentially DR ranks 29 and lower coming to your website use the free Google disavow tool to notify them these links are not recommended.
A disavow list is just a big list of all those backlinks you do not want coming to your site compiled onto one sheet.