An L1 is your mother tongue, the first language you learn in your home environment. L2 has various meanings. It can refer to any language learnt after learning L1.
It also refers to the language learnt after the L1 and that is used in the learner’s environment (e.g. learning Greek as a child while living in Greece, having first learnt English from your English-speaking parents).
A third meaning is for languages widely used in countries or regions but not recognised as official languages. For example, in Guyana, English is the official language but Guyanese Creole is an L2 widely used by many people.
"Nowadays, with so many people being bilingual, it is not always simple to say which is their L1 and which is their L2."
Cook, V.J., Long, J., & McDonough, S. (1979), First and Second Language Learning, in G.E. Perren (ed.)The Mother Tongue and Other Languages in Education, CILTR.
Crystal, D. (2003). English as a Global Language. Cambridge: Cambridge University Press.
Ellis, R. (1985). Understanding Second Language Acquisition. Oxford: Oxford University Press.
Kachru, B. (1992). World Englishes: approaches, issues and resources. Language Teaching, 25. Cambridge University Press.
Kachru, B. B. (1997). World Englishes and English-using communities. Annual Review of Applied Linguistics, 17.