Relatable is an adjective of comparatively recent coinage, only entering most people's vocabulary in the 1980s. However, since that time is has become a fairly standard English word, to the point that many people are surprised to learn that it is a neologism. It has two commonly recognized definitions, although the second is by far the most common nowadays.
1. That which is able to be narrated or retold. The first recorded use of this word was in 1825 (specifically referring to speech that was not relatable, due to improper language).
2. That which is easy to understand because it is familiar. This usage is much more recent, first appearing in educational theory material in 1947. It gained momentum in the new-age speak of the 60s, and fully entered popular usage in the 80s.