Google Now is your personal assistance which gives notification to you, if you had Android smartphone then you must have noticed cards from Google Now regarding weather or sports.
If you had signed in to your Chrome with same account which you are using on your mobile then you will be able to get notification on desktop too. If you don’t want to get notification and looking a way to disable it then you could do it by simple tweak.
Temporary Disable :-
If you want disable it temporary then just right click on a bell icon placed on your taskbar and select any one of the listed options:-