“16 bars” is a very familiar slang for rappers and hip-hop artists. Technically speaking, it refers to the 16 measures used in the making of music. However, the “16 bars” term has been very much simplified in rap and simply implies a verse or chorus inside a rap song.
“Eminem’s 16 bars are always nasty because of the intensity.”
“I felt like Kanye West’s new 16 bars are soft and weak.”
The slang term is still widely used today. It is used when teaching rap or when referring to some other stuff related to rap. It has also been used in a documentary about rap.