The phrase is often used in a playful or humorous way to express that something is obviously Malayalam, either in terms of language, culture, or anything else that is characteristic of Kerala.
For example, you might say "Malayalam thaan podo" if:
* Someone speaks in a thick Malayalam accent.
* You see someone wearing traditional Kerala clothing.
* You hear a song in Malayalam.
It's a way of acknowledging the distinct features of Malayalam and its culture.