Focussing is all about making sure you have your subject nice and sharp in the picture.
The human eye is really clever and shifts focus all the time. When we have a close up object and a distant one we can switch focus between them. It looks as though we are seeing detail in both at the same time. In reality, we only have one of them in focus at a time and our brain is putting the two together.
Your camera cannot do that. When you take a picture the whole scene is recorded at one time. Whatever is focussed on the digital sensor is sharp and crisp. Whatever is not in focus in will appear soft and blurred.
DSLR cameras are pretty good at getting things in focus very quickly. If you have used a compact camera or your Smartphone camera you will have noticed that it can take a while for them to focus before you can take a picture.