The “Day Grid” is the grid that shows up in your Calendar when you are looking at your Calendar in “Day” view and "Week (Column)" view. The day grid will show times in regular intervals for a specific length of time.
The "time-of-day" indicator is a red line in your calendar with a small clock icon that shows you what time of day it currently is when you view your calendar with the Day Grid turned on. See the image below:
If you want to turn this feature off, you can go to the "Customize" screen for your calendar to disable it. Follow the steps in the image below: