The grammar is the package (normally called
language-*) the provides information about the text in a file. One use is syntax highlighting, but there are other applications like code folding, indentation, autocompletion, etc., that all can benefit from this information.
What happened is Atom is moving to a newer way to write grammars. The new way (Tree sitter) has potential to be much more useful for this kind of thing, and is also faster than the current (TextMate) grammars (though you probably won’t realise unless you have massive files / long lines).
Currently, you would not notice much of a difference. However, as more work is put into supporting and developing Tree sitter grammars, you will miss out on these new features and improvements. Fixes to TextMate grammars will likely be neglected, unless a community member provides the PR, it’s serious, or it’s a simple fix.
The most recent update (1.32.0) switched Tree sitter to be enabled by default (it was present in several earlier versions, but disabled by default).