Hey, First. How 'bout you go on over to the Introductions forum and tell us about yourself.
The slide stop lever only engages when there is a magazine inserted AND the magazine is empty. Some people use an empty mag to lock the slide back.
Somebody here will elucidate on the actual function, but each pistol platform functions a little differently, of course.
Jpyle already elucidated...
.45acp, .40 S&W, 9mm, .38 Spl, .380 acp, 22lr
12 gauge, .223/5.56x45mm, 30-30 Winchester
2 Chron. 7:14 Christians must seek His face