I use macOS and have done so for several years. But I had to look up this app icon to know what app it was. It’s the Pages app, which I don’t use and don’t keep in my dock. Looking at only the leftmost icon, I was thinking it might be the Notes app or the Freeform app, both of which might conceivably also be represented by what to me looks like an Apple Pencil for iPad.

Looking at the reminder of the icons, I recognize that it’s not the Notes app because although I no longer use that one I have in the past so I remember that it has looked like a notepad with some lines and some yellow on it. But the leftmost one might as well have been a newer version of Notes than the one I last used.