With digital photos, your work doesn't end when the photo is taken. While Photoshop is the most famous photo processing software package, there are dozens more can be used to alter and manipulate photos. With practice, you can use them for a lot more than pasting photos of your little sister's face onto zoo animals.
Beginners can use post processing to adjust brightness, contrast and color balance. This is an easy way to make a poor photo passable and make a good photo look great. More advanced users can play with color channels, apply special effects or make a composite of multiple photos. For example, if you were bracketing a shot of a dog in front of a window, you might get one photo with the dog properly exposed but the window too bright, and one with the window exposed but the dog too dark. In post processing, you could combine the two for one perfectly exposed image.
While there's no limit to the creative things you can do with post processing software, there's a reason this term is last on the list. Learn to shoot excellent photos with the camera first. Then you'll have the raw materials to do good post processing work later.