L1 is a speaker's first language. L2 is the second, L3 the third etc.
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.