Setting the value: Select a point (keyframe) on the curve and add the time and the value of it below the grid or simply drag it in the curve editor. Hold Shift to restrict to horizontal movement (adjusting time only).

Adding a keyframe: Double click in the grid or right click and select Add Key to add a keyframe.

Determining the flow between keyframes: Each keyframe determines the interval from the keyframe til the following keyframe. By default all keyframes will be curve points when you add them, but you can easily change the type:

 Stepped:
This will keep a fixed value until the next keyframe in the timeline is reached.

 The stepped keyframe will by default not have a tangent handle opposite the stepped section, but you can add it via the Tangent handle icon. This way you can ease into/out from a stepped

 Use the value at the start of the interval.

 Use the value at the end of the interval.

 Curve:
Curve keyframes have tangent handles that can be manipulated to make the transition ease in/out.

 Lets you enable/disable the left/right tangent handles

 Keep tangent handles locked opposite to each other.

 Unlock tangent handles.

 Straight:
This is a variant of the curve point, where both tangent handles have been disabled by default. Simply click the tangent handle icons to make it a regular curve point.