In this part of Pilgrim's Progress, Christian finally arrives at the cross. He has a hard time getting there because his burden was very heavy. When he arrives at the cross, however, his burden disappears.
He ran thus till he came at a place somewhat ascending; and upon that place stood a Cross, and a little below, in the bottom, a sepulchre. So I saw in my dream, that just as CHRISTIAN came up to the cross, his burden loosed from off his shoulders, and fell from off his back, and began to tumble; and so continued to do till it came to the mouth of the sepulchre, where it fell in, and I saw it no more.
Then was CHRISTIAN glad and lightsome, and said, with a merry heart,
"He hath given me rest by his sorrow,
And life by his death."
The significance of this parallels the experience of Christians who are saved. Remember that this work is a Christian allegory. The burden that we all carry on our backs is sin. We cannot rid ourselves of sin. Only Christ can take away our sins. The reason that Christ died on the cross was to pay the penalty for the sins of mankind. So when Christian gets to the cross, his sins disappear. The cross is a symbol of salvation, a symbol of eternal life.
See the analysis here on enotes.