There are two types of graphs in the Ae graph editor. You’re probably used to the value graph, which shows the value of the property on the Y axis against time on the X axis. This is called the speed graph, and it shows the rate of change of the value of the property on the Y axis against time on the X axis.
Value graph shows you how the value itself is changing over time; the speed graph shows you how its speed is changing over time.
Here’s the official documentation:
https://helpx.adobe.com/in/after-effects/user-guide.html/in/after-effects/using/speed.ug.html
And here’s a very nice explanation from School of Motion:
https://www.schoolofmotion.com/blog/graph-editor-after-effects