This is an example of a medieval English song that has not survived the translation into American English.
Rose, rose, rose, rose
Shall I ever see thee red
Aye, marry, that thou wilt,
An thou but stay.
Aye means 'yes' and at the time of the Wars of the Roses, when the red rose represented the royal house of Lancaster and the white rose was York, 'marry' meant 'certainly' and 'an' was 'if.'
In modern English, then, the last two lines mean 'Yes, certainly you will, if you only stay.'
The song was probably meant to echo the words of a Lancastrian Tudor when the Yorkists were in power.