What does it mean?

They are 3Dimensional pictures of trees/plants placed on 2D planes.This is a means of using less polygons and reducing rendering time.I recommend them highly especially in Bryce.They do throw a ground shadow BTW.

PS This means you can only look at them front and back.Looking at them sideways brings the expression "I've seen more fat on a sparrow's ankle" to mind.:biggrin:

what does what mean?

Billboards are large rectangular structures carrying an image, a poster, a photo.

In 3D, a billboard polygon is a rectangle (or other polygon) that carries a picture and is placed into a 3D scene in order to carry and show that picture without the need for creating a real 3D model version of the same. For example, in a city street scene you could just see yourself texturing a poster image onto a large rextangle that's 'hanging' on a brick wall, to embellish it with today's top news from a newspaper ad, whatever.

More typically, a billboard polygon carries an image with transparency from an alpha channel. That is often used then for realistic foliage, trees, plants, etc... which are either painted (e.g. with PD Particles) or with from Photos that have been worked on to make them stand on transparent backgrounds. So you end up seeing the solid inner side, the plant, and the rest of the scene behind is not hidden by the rectangle's borders.

You can see several examples of plants created in PD Particles at
http://www.thebest3d.com/pdp/tutorials - used in Carrara, Poser, Bryce, Blender and other 3D systems. Doesn't make a difference whether you use PD Particles or Photoshop or Paint Shop Pro or whatever, or whether you buy collections of images like the Xfrog Trees and plants. You can even make your own in a 3D tool like Carrara, and render them with transparent masks or with keying to color later. see for example www.thebest3d.com/carrara/plants
(the first part of it anyway)
I hope this helps understand what a billboard polygon is.

Oh, here's another example, with Carrara:
http://www.thebest3d.com/dogwaffle/whatsnew/3_5/optipustics_new.html
(scroll to the middle and bottom)

-Philip

PS: if you didn't know, XFrog makes image collections of plants

So billboards are just pics? got it. thanks guys. 

"I've seen more fat on a sparrow's ankle"  :biggrin:

 

strictly speaking billboards are the combination of a rectangle polygon (usually) in the 3D scene and a picture image usually from a file (often with alpha masking). In this context, they provide the pictures. The rectangle part is up to you, usually, but it is conceivable that they include a sample uv-mapped and textured rectangular polygon (or two triangles covering the same).

Here's another example:
http://thebest3d.com/pdp/tutorials/blender/criss/09_bridgeoflove.jpg

the various tufts of tall grass are done on billboards.