When cutting a piece of wood, the two boards are an identical mirror image of each other.

The router makes these lines during the flattening process as it cuts away the wood. Fix this by sanding.

The floor sander uses 40 grit to eliminate the router marks if they don't fit in my 20" planer. This is normal and fixed by sanding with finer grits.

This wood is rotten and now 'punky.' You can remove and fill with epoxy, or solidify it with urethane.

Some wood species and/or grain direction cause the planer to do this, especially if the blades are getting dull.

As the wood dries, it shrinks, and when it splits along the growth rings, it's called ring shake or wind shake. Not easy to repair.

a fungus enters a dead or dying tree and causes these lines. If caught before it turns punky, it adds great beauty while retaining the wood strength.