What is the effect of the figure of speech used in the sentence the dancer glided like a swan?

The figure of speech used in this sentence is a simile.

A simile compares two things using the words "like" and "as"

In this sentence, the simile is being used to provide the characteristic of the dancer as to how they are gliding.

In other words, the simile is describing how the dancer was gliding.

Extra:

A simile may at times be confused with a metaphor which is the same thing as a simile however metaphors do not use the words "like" and "as".

glamis thou art, and cawdor; and shalt be what thou art promis'd; yet do i fear thy nature; it is too full o' the milk of human kindness to catch the nearest way: thou wouldst be great; art not without ambition; but without the illness should attend it. what thou wouldst highly, that wouldst thou holily; wouldst not play false, and yet wouldst wrongly win: thou'dst have, great glamis, that which cries, "thus thou must do, if thou have it: and that which rather thou dost fear to do than wishest should be undone." hie thee hither, that i may pour my spirits in thine ear; and chastise with the valor of my tongue all that impedes thee from the golden round, which fate and metaphysical aid doth seem to have thee crown'd withal.  

he lacks ambition.  

he is hesitant.  

he lacks courage.  

he has moral scruples.

alrigt gdy