Theory:

Refrain is a type of repetition found in poetry. In this type of repetition, a phrase, a line, or lines are repeated instead of a word.
Pronunciation Guide:
Refrain -- re (as in "remember") -- fra (as in "phrase") -- in (as in "pain")
According to J.A.Cuddon, refrain is "a phrase, line or lines repeated at intervals during a poem and especially at the end of a stanza". 'Very often it is an exact repetition, but sometimes it will undergo a slight modification'.
As Cuddon pointed out, refrain can also happen with modification. Let us take the following poem "Disdain me Not" by  Sir Thomas Wyatt as an example:
Example:
Disdain me not without desert,
Nor leave me not so suddenly;
Since well ye wot that in my heart
I mean ye not but honestly.
Disdain me not.
 
Refuse me not without cause why,
Nor think me not to be unjust;
Since that by lot of fantasy
This careful knot needs knit I must.
Refuse me not.
 
Mistrust me not, though some there be
That fain would spot my steadfastness;
Believe them not, since that we see
The proof is not as they express.
Mistrust me not.
 
Forsake me not till I deserve
Nor hate me not till I offend;
Destroy me not till that I swerve;
But since ye know that I intend,
Forsake me not.
 
Disdain me not that I'm your own:
Refuse me not that I'm so true:
Mistrust me not till all be known:
Forsake me not ne for no new.
Disdain me not.
The above contains several other types of repetitions as well. However, what is to be noted here is that this poem contains two sets of refrains.
  • The first set of refrains can be seen in the individual stanzas. Each stanza has a refrain that appears in the beginning and the end. For example, the second stanza contains the refrain "refuse me not" at the beginning and in the end.
  • The second set can be seen in the poem as a whole. Throughout the poem, the phrase "disdain me not" is repeated with modification. Words such as refuse, mistrust, and forsake are used as modifications here.
Hence, if a line or a phrase is repeated, it is called a refrain.
Refrain from the poem "The Quarrel":
Example:
I quarrelled with my brother
I don’t know what about,
One thing led to another
And somehow we fell out.
The start of it was slight,
The end of it was strong,
 He said he was right,
I knew he was wrong!
  
We hated one another.
The afternoon turned black.
Then suddenly my brother
Thumped me on the back,
And said, “Oh, come along!
We can’t go on all night —
I was in the wrong.
So he was in the right.