Linguistic distance (also language distance) is how similar or different one language or dialect is from another. In our case, we're mainly interested in distance between English and our students' L1.
Languages are related to each other because of language evolution.
Languages that are very close to English are other Germanic languages, particularly Afrikaans, Danish, Dutch, German Norwegian and Swedish. French is an interesting case, as it is close to English lexically but somewhat less close grammatically. A language like Hindi is also an Indo-European language, but rather distant. Whereas Mandarin is vastly different.
The further distant the student's L1 is from English, the harder English is for those speakers. It shouldn't be too surprising then that Dutch native speakers are often very good at learning English, whereas Chinese have lots of difficulties.