Where are you looking at
Your lens is designed to gather light from your object, but you need to adjust your lens to the correct distance if you want to see it properly. The more light you bring in (the smaller aperture) the more concentrated you will be around this distance.
When you take a picture, you gather light coming from the scene you are framing. Good lenses will allow you to see your object very clearly and capture the light around your object very well (we talk about lenses with small aperture). However the trade-off is that, they can only collect this from a small region (depth of field). Things that are closer or more distant will not be as clearly defined (or sharp) they will appear blurred. You can however adjust this distance and determine at what distance you see things sharp. This is called focusing.
The focus is an important tool for a photographer, when used properly it will tell the viewer where to look at. The flip side is that, if you are not careful, your pictures will not be sharp.
Most modern camera/lens systems will allow you to automatically find the correct focus. The default setting is that the camera will autofocus when you press the shutter button halfway. As such it is also part of taking a picture. But your camera can not read your mind, it needs some help.
By default the camera would focus in the center of the frame. But modern cameras have many ways to manipulate this. There are focus select points that allow you to select which part of the picture you want to focus. You can usually manipulate them with directional buttons on the backside of the camera.
There are several tricks with autofocus:
While focus allows you to tell a story, sometimes you may actually need to broaden the depth of field so that you can get more in your shot. For this you need to decrease the aperture. The following two shots use a relatively small aperture f/5.6 but you can see that we either focus on the nose, or on the eye. At this aperture focusing becomes an issue, and is maybe not what you want.
If you have to choose, most of the time, the eye will work better. Especially when you get close to your subjects be prepared to reduce the aperture
These pages are for Amateur Photographers and not really for seasoned photographers and professionals. I have no affiliation or commercial interest with any brand/make. I write from my own experience. I ended up using mainly Nikon, so I am more familiar with this brand than others. See price for notes on pricing as well as photography related links.