What does [msg] mean in the primitive block?

I wan to make a language category using the block:
untitled script pic (2)
What does '[msg] mean in the option dropdown?

Just don't.Use the library blocks that have an encapsulation.

I am a new snapper, so what is an encapsulation?

Do not enable :gear::ballot_box_with_check:Extension block, until you know for sure you need it.

You cannot add a new language to Snap! this way. You must modify the source code on Git Hub. I would not recommend messing with the extension blocks unless you know what you're doing.