In simple terms, Karela is the original name from the Gujarati language that refers to a very bitter fruit called the bitter gourd,or bitter-melon.
In the literary sense, this term is found in the second Jungle Book, specifically in Mowgli's "Song Against People".
In this song, Mowgli basically puts a spell on the tribes-people by chanting the verse:
I will reap your fields before you at the hands of a host;
Ye shall glean behind my reapers, for the bread that is lost,
And the deer shall be your oxen
By a headland untilled,
For the Karela, the bitter Karela,
Shall leaf where ye build!
I have untied against you the club-footed vines,
I have sent in the Jungle to swamp out your lines.
The trees--the trees are on you!
The house-beams shall fall,
And the Karela, the bitter Karela,
Shall cover you all!
In these last two stanzas we see that the Karela is not a very appetizing fruit, nor its vines are desirable to have around. That is basically what a Karelais, and what its contextual meaning is as well. In modern terms, Karela is also a fruit used in alternative medicine to treat diabetes.