If you compose your shot like this and focus as normal, your camera will try to guess what needs to be sharp. Most cameras consider the centre of the picture to be the most important part, and focus on it. This will result in the background being sharp, but the woman in the foreground being blurry.
Clearly this isn’t what we want; we want the woman to be in focus seeing as she’s the most important part of the shot. This is where focus lock comes in.
To use it, point your camera at the subject and half press the shutter button. You should hear a beep and see a light come on in the viewfinder to let you know the camera has focused. The focus will now remain locked while your finger is still holding the shutter button half down.