The Render Weight of an avatar, prim, or any mesh object represents the amount of work your computer must do in order to display an avatar or object. It does not affect land impact, but high render weight values may result in low visual performance on some computers. The Second Life™ viewer has three zones for avatar render weight, green, yellow and red.  I have not found an official document that defines the render weights for the green, yellow, and red zones but after experimenting a bit green is about 0 - 15,000, yellow about 15,000 - 40,000, and red above 40,000.  A couple scripts in Dark Radiance monitor all avatar resources, render weight is one of the resources monitored, in addition to script count, script time, script memory, and avatar height.  

Most avatars will find it very difficult to be in the green zone and or the yellow zone of the calculated render weight.  The scripts at a Dark Radiance will present your avatar with a dialog if you avatar has a render weight over 70,000. This is over 30,000 points into the red zone, so it seem fairly reasonable.  I wrote this script to make people aware that their avatar could be causing lag on their and other's computers.  If I get enough complaints I will turn this feature off, but I hope that people find it useful.  BTW, my avatar typically has a render weight between 28,000 and about 56,000 depending on my outfit.    

If all avatars reduced their render weight to reasonable levels, Second Life™ would have less lag.  Here are some suggestions to reduce your avatars render weight (taken from the "Render weight best practices for Second Life" on Avataric).
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1. Check your render weight.
2. If you’re confident doing it and your render-heavy items are modifiable, follow some of Penny’s suggestions about ripping out root prims, unnecessary layers, and scripts.
3. Ask the creator about render weight before buying. Wouldn't it suck to buy a gorgeous attachment, only to discover that nobody can see its glory because it’s too heavy to render? Before making a significant purchase, send a note card to the creator and ask about render weight and script usage.
4. Ask your favorite creators to add render weight to their product information.
5. Ask your favorite SL fashion bloggers to add render weight to their photo descriptions.
6. Don’t count on mesh to be efficient. Well-made mesh will be lighter than sculpties, but mesh alone isn't a guarantee.
7. Don’t equate high detail with high render costs. 
8. When cleaning out inventory, check render weight as part of your filtering process.
9. When you save an outfit, include its render weight in the description.
10. Make and save some outfits with low render weight and low script usage for visiting busy places.  

For in-depth information on render weight and some articles see the following we resources:

http://wiki.secondlife.com/wiki/Avatar_Rendering_Cost  
http://wiki.secondlife.com/wiki/Mesh/Rendering_weight
https://avataric.wordpress.com/2015/02/27/render-weight-basics-for-second-life/
https://avataric.wordpress.com/2015/02/09/render-weight-best-practices-for-second-life/

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