F-curves are curves that show the changes in value over time of animated properties. Use this tag for questions about creating and modifying F-curves.
The manual describes F-Curves thus:
After animating some property in Blender using keyframes you can edit their corresponding curves. When something is “animated”, it changes over time. This curve is shown as something called an F-curve. Basically what an F-curve does is an interpolation between two animated properties. In Blender, animating an object means changing one of its properties, such as the object’s location, or its scale.
Interpolation is a mathematical term for the process of calculating what animators call in-betweens, the value that an animation property takes as time moves from one keyframe to another.
F-curves are changed by changing keyframes, but they can also be manipulated in the Graph Editor.
Use this tag for questions about creating and modifying F-curves, especially in the Graph Editor.