Loli is a term that is used to designate mid-pubescent teenage girls or a pre-pubescent pre-teen girls, depending on the situation. A person who is attracted to lolis is called lolicon. Lolis are a topic which sometimes comes up in the incel community.
The term derives from 'Lolita', a nickname for the fictional character Delores Haze (age 13-19 in book), from the eponymous novel by Vladimir Nabokov. In Japan, the term "complex" is often used to refer to an obsession or sexual fetish, so the name was shortened to "Loli" and the suffix -con was added, creating the term "Lolicon", which described a person with such a fetish.
In its basic form it refers to peripubescent girls. Over time its usage has broadened to also include pre-pubescent pre-teen girls in some situations. Some prefer that added usage so much they even abandon the original meaning and ignore the word's root.