L1 stands for first language - ie the language that a speaker acquired naturally as a child. This is also often referred to as the speaker's mother tongue. Both these terms are open to criticism as a) in a bilingual situation, someone may acquire more than one language as a child and therefore have more than one L1; the first language may not be acquired from the speaker's "mother" but from the father or other carer, or from the society as a whole.
However, those caveats accepted, the concept of L1 contrasts with that of L2 - any second (or further) language which is not acquired by natural exposure during childhood, but which the person learns/acquires at a later stage.
The term second language is, however, sometimes used contrastively with the term foreign language. Used in this way :
- second language = a language which the person acquires/learns for use within the community in which they are living. For example, immigrants to the UK might need to learn English as a Second Language.
- foreign language = a language learnt for purposes unrelated to the community in which the learner lives. Eg an Italian learner may study English as a Foreign Language as an academic subject at school or, if s/he then goes into business, use it as a means of communication with non-Italian clients.