Example?
If you make a claim like there is a dragon in my garage. Then we would expect to see a dragon if we go into the garage. If we don't, then this is evidence against your claim.
Another trivial example, is when the fire service are called to put out a fire. When do they leave?
When there is no evidence of a fire surely.
This is just another way of stating the first example. If there were a fire you would expect to see A,B,C etc. If you do not see A or B or C then this is evidence of the absence of fire, and so they can leave the scene.