It is figurative in this context. In society or an organization the most succesfull people are at the "top" and the least are at the "bottom". Example: "started from the bottom" - common expression. Immigrants typically enter a society with less oppurtunity than a citizen so they are considered among the "bottom". And they usually work jobs considered too "low" for citizens. Actually using "bottom" in this context is quite common and you can use it to talk about many things, like philosophy, politics, heirarchy and is useful in formal and informal contexts (but not professional).

It means you are at the lowest level or just starting.

It means you are at the lowest level or just starting.