What is the meaning of "Of + ing"?
To start off, the word "of" functions as a preposition. Prepositions have several different functions; in this particular clip, it shows the relationship between the person and the action. When Olaf sings "Of relaxing" it is showing that relaxing is what he is currently doing. However, most sentences do not start with the word "of." This sentence seems to be a continuation of the previous fragment. The whole sentence would be "When life gets rough, I like to hold on to my dream of relaxing in the summer sun, just lettin' off steam."