II.off [美 ɔf, ɑf, 英 ɒf]副off often appears as the second element of certain verb structures in English (break off, pay off, take off, etc). For translations, see the relevant verb entry (break, pay, take, etc).
I.on [美 ɑn, ɔn, 英 ɒn]介词on often appears as the second element of certain verb structures in English (count on, lay on, sign on, etc). For translations, see the relevant verb entry (count, lay, sign, etc).
The lyrics describe a couple in love lamenting about being too young to run off to get married, fantasizing about how nice it would be if they were adults.