Fat laces are like regular laces, but a lot wider. They are most commonly used in combination with skateshoes, although they are also used in shoes that are not skate-specific but worn alot by skaters, like Converse Allstars. Fat laces are seldomly tied, most of the time they are just tucked away in the shoe.
Fat laces often have bright colours and colours that are contrasting with the shoe they are in. Left and right don't have to be of the same colour, too.
"Check out my new pink fat laces!"
"Dude, they are awesome! They must be an inch wide."