You probably already know, that to make a capital letter, you hold the Shift key down while you punch the letter.
If you punch the "Caps Lock" key, on most keyboards, it turns a light on, on the keyboard. Then whatever "letters" you type will be capitalized. However, if you type any numbers (with the keys above the letter keys), they will still be numbers (unlike with the "Shift Lock" key on a typewriter). And punctuation keys also still work the same with Caps Lock on or off. This is very handy.
When you are writing an e-mail message to someone, it has become a custom that putting a word or two in all capital letters, is used for yelling (except in titles). For example, THIS IS YELLING. Because many people know this, if you put a word or two in all capitals, or even worse if you put a whole message in all capitals, it can be annoying to read.
If you send out e-mail in all capitals, you may find people suggesting you turn the Caps Lock off.
I once saw an e-mail done in all capital letters from a technician, that was sent to a group discussion with many technicians. The technician was asking a very technical question about installing a hard drive, and obviously knew a lot about computers. Someone jokingly replied, "I think you should learn how to turn the Caps Lock off first!"
So remember, keep that Caps Lock turned off, and only use it if you really mean to YELL!