The "Ripple" Filter allows you the distort the image to look like a disturbed water surface.
Checking the Antialiasing box makes jagged lines at the edge of the ripples be smoothed out without blurring the image.
The Retain Tileability ensures that where an image is tileable, such as a pattern, the final distorted image will also be tileable — i.e. you can place copies of the image side by side to create a continuous image without any break.
Defines which way the ripples occur — either horizontal or vertical.
When you apply a ripple effect, it performs the distortion within the confines of the selection, and so there will be small areas at one edge of the selection which are missing.
Selecting Black fills in these area with solid black. Smear will fill this area with pixels stretched to fill and Wrap fills the blank area with the pixels that were lost from the other side of the image.
The wave type defines how the curve of the wave is calculated. You can choose either Sawtooth which makes a sharper wave or Sine which makes it softer.
Setting the Period sets how long the waves are — the distance between two crests. Amplitude defines how high the wave is — the vertical distance between the mean and a crest.