The clone stamp tool is one of the most useful tools in photoshops arsenal and so it is one of the most used. It is used for retouching and as the name suggests, the tool clones one area of an image and stamps it onto another area. Images are made up of multiples pixels and what the clone stamp does is basically replicates an area of pixels and pastes it over the part of the image you want to retouch. Since the tool replicates pixels, it is almost never noticeable that an image has been retouched.
First, select the clone stamp tool by going to window>tools and then select the stamp tool. You can change the brush settings(Brush size, shape, opacity, flow, and blending modes) from the bar at the top of the screen according to your needs but to have the tool replicate the pixels exactly then leave the settings as default. Now, to select the area you want to clone, place your cursor over the desired area and press alt+click(windows) or option+click(mac). This will create a point which moves along with your cursor. Now start clicking over the areas which you want to cover up and you will see that the selected point moves along with the cursor and the masking looks seamless.
Be sure to change the point which clones the area as required by again pressing alt+click or option+click and also the brush settings. If done properly, the masking will look as though it was the original image all along!