A timeline level grade is where you apply a node to the entire timeline instead of just a single clip in your sequence. This can be an incredibly useful tool if you need to quickly affect the whole sequence.
This could be useful if you decide you want to desaturate the project, add some overall sharpness or apply a LUT to your grades.
To create a timeline level grade the first things you need to do is select Clip and change it to Timeline.
Clip Level Grade
Timeline Level Grade
Now, when you are in the timeline node tree you can affect the overall image of the entire project with just a single node.