Hi there and welcome justmyheart1989
When you depress the shutter-release button on a digital camera, focus and exposure are locked at the same time. Others have settings that enable the photographer to lock focus and exposure independently of each other. We have information at the main site, which explains techniques of locking focus and/or exposure on a subject.
Controlling focus and exposure together
http://www.digicamhelp.com/shutter-release-button/index.htmControlling focus and exposure independently
http://www.digicamhelp.com/focus-exposure-lock/Let us know if you have additional questions.