Modal editing is an older concept, where by-default you can't hop in and start typing like normal. Most modal editors (including Helix) start you in a Normal mode, which is mostly used for reading and searching documents. Then you press a button (usually i or a) and get put into an Insert mode, where you can edit the text and type to your heart's content. Pressing escape will drop you back into Normal mode when you're done, where you can save the document and exit the editor.

It's a different workflow, but kinda nice when you want that IDE-style experience in a simpler interface like a TUI.