Google Keep Notes has arguably been among the company's more reliable and streamlined apps. It (as of recently) does exactly what its name suggests, and does it well - taking notes of various flavors including voice-based ones, making lists and setting reminders.
It's now seeing a rather significant visual overhaul in the vein of the company's Material Theme paradigm: the bright yellow bar up top is gone, as is the gray background on which the note cards rest. A simple all-white background with a floating search bar at the top is what we get in its stead.
Other tweaks are also quite up-front; the note cards are now more rounded, and their header text now makes use of the Google Sans font; the body text typeface is no longer the classic Roboto Slab. The account switcher that one would normally find at the top of the hamburger menu has now been relegated to a button at the far-right of the search bar. The hamburger menu itself is otherwise largely unchanged, outside of typography and iconography tweaks.
Oddly enough, none of this has warranted much of a bump in the app's version number; the newest version prior to the redesign was v5.0.391, whereas this shiny, Material Theme-d update for it is v5.0.411.
Even bigger - and arguably more exciting - changes are supposedly coming in future updates: a (verified) Googler in this Reddit thread has revealed that a dark mode and markdown support will eventually make their way to Keep Notes.
As for the update at hand, it's currently seeing a gradual rollout, so it should show up soon for you; until then, keep hitting that "check for updates" button. If you don't already have the app, iOS users can download Google Keep Notes from the App Store here, and those with Android handsets can get it here from the Play Store.