Flicking shadows on terrain can often be fixed by adjusting the "Bias" and "Normal bias" of the light source. However, adjusting those settings does have side effects, so you should pay attention to how the settings affect your shadows in different areas (especially on high-detail objects close to the camera). In the worst-case scenario, you might have to use tricks, like rendering nearby objects with one camera/directional light, and distant objects with a different camera/directional light, to get close and distant objects to both look right.
Other possible causes of flickering shadows:
- The camera's near clipping plane is set too low (it should usually be at least 0.3 and may need to be set higher if the far clipping plane is very large).
- The camera is very far away from the world center (more than 100,000 units if I remember correctly).
Baked lighting doesn't cast real-time shadows (e.g. character shadows) because it's baked. Baking your lighting essentially renders the shadows into a texture that is applied on top of the meshes. For traditional baked lighting, there's very little performance cost but it's completely static. However, you can combine real-time and baked lighting using mixed lighting.
Baked and real-time lighting are important concepts, so if you weren't already familiar with the differences between them, you should take some time to thoroughly read the Unity documentation on the different lighting modes.