Some translations set poetry farther to the right than the rest of the text to show that it is poetry. The ULB does this with the poetry in 1:2-3, which is a quotation from the Old Testament.
Leprosy was a disease of the skin that made a person unclean and unable to properly worship God. Jesus is capable of making people physically "clean" or healthy as well as spiritually "clean" or right with God. (See: [[rc://en/tw/dict/bible/kt/clean]])
Mark begins this book about Jesus Christ with the words that the prophet Isaiah wrote long before that time. Then he tells how John the Baptist and Jesus Christ fulfilled that prophecy.
Repent means stop sinning. John the Baptist taught people to repent so that God would forgive their sins. Jesus taught people to repent and to believe the good news about the kingdom of God.