L1 is also sometimes referred to as "mother tongue".
Example
A learner whose L1 is Spanish may find Portuguese and Italian easy languages to learn because of a fairly close connection between the languages.
In the classroom
L1 interference - where a speaker uses language forms and structures from their first language in language they are learning - is an area many teachers are concerned with. In a mono-lingual class where the teacher also speaks the L1, it is easier to identify interference and address it, often discussing it explicitly with learners.
Further links:
https://www.teachingenglish.org.uk/article/mother-tongue-other-tongue
https://www.teachingenglish.org.uk/article/translation-activities-language-classroom
https://www.teachingenglish.org.uk/article/planning-a-writing-lesson
https://www.teachingenglish.org.uk/article/how-should-learners-own-languages-be-used-classroom