It's a four inch wide piece of heavy duty webbing, which is latched around two trees and pulled fairly tight. Then you (attempt) to balance not fall off. It has some give and tends to shake when you are on it.
But it is fun!!
MrMom has two boys, Enviro and Fearless. Fearless is just a toddler, so sometimes MrMom has to balance Fearless with helping out a kid or two on the slackline.
On the last day of school MrMom brought Enviro and Fearless to the park along with his slackline. And we made a giant "V" shape amongst the trees.