When you set two keys on a property, the value in between those keys is automatically calculated. This is called interpolation. Interpolation settings can be customized to create dramatically different results.
You can set the easing on your keys by either using the Interpolation panel to the right of the Timeline, or by using the Graph Editor, which you can toggle on via the shortcut near the Timeline options.
Using the Interpolation Panel
The Interpolation Panel appears to the right of the timeline when you select any number of keys on the timeline.