Actually you have showed not brush tool but healing tool which is quite different thing. The healing tool tries to copy a texture from sample area to the target area trying to keep original color at the target area.

Let's say you're using a peace of clean fabric as a source and trying to "cure" the same fabric but with a spot on it. Healing tool will take into account the slight color and brightness difference and try to use the texture from the sample.

By the contrast the clone tool will copy not only the texture but the colors and brightness resulting in rough boundaries of the target spot even if there is a slightest color or brightness difference to the source.

My recommendation is to try yourself to experiment with some images with brush, clone and healing tools. Be advised that two last tools requires that the sample area should be pointed (do it with Alt button).

As @MikeW mentioned in the comment it is a good idea to search Youtube videos for any Photoshop tool you will probably need. Or about any process like correct geometry, remove spots, white teeth or eye whites or almost anything you could ever need. There is also plenty of tutorials which could explain you many Photoshop principles such as adjustment layers, channels and many others.