1968 was a watershed year for Johnny Cash. He beat his drug addiction, he married his wife, June Carter, and he recorded this album, which brought him to the top of the Country Album Chart (#1) and into the Top 20 Pop Album Chart. With the success of this album, released in May 1968, he went on to record a follow-up album "Johnny Cash At San Quentin" in 1969, which gave him his biggest crossover hit, "A Boy Named Sue."
Of course, before the concert in Folsom there was the song, "Folsom Prison Blues." It was written in 1955 after Cash saw a documentary on prison life. It didn't do much when it was initially recorded, but was re-released in 1968 with the album and hit Number One on the country charts.
I couldn't find any actual footage of the Folsom Prison concerts, but here is a scene from the mid-Seventies of Johnny Cash performing at another prison, this time in Tennessee. And of course, he is performing "Folsom Prison Blues."