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".

either a donut or a sphere or a pancake or a flat sphere. it depends on where the vertical line is.

if the line goes through the center of the circle and is perpendicular to a diameter containing the center, then you get a flat circle.

if the line and the diameter of a the circle meet in the center of the diameter and at an angle, you get a pancake.

if the line is on the circumference of the circle and is at right angles   to the circle you get a ring.

if the line contains the diameter, you get a sphere.

the the line is parallel to the diameter and touches the circumference, you get a donut.