Oxalate is an antinutrient and a toxin. It is an antinutrient because in its insoluble form it readily binds with calcium, magnesium, and iron to make oxalate salts (such as calcium oxalate) which are insoluble and usually pass out through your stool. This keeps you from being able to absorb the vital calcium and magnesium. Diets high in oxalate and low in calcium are linked to low bone density. You can either reduce your oxalate consumption (e.g. eat more kale and mustard greens and less spinach) or you can increase your mineral consumption (especially in meals with low oxalate content) to decrease the antinutrient effect.
Oxalate is also toxin when it enters your bloodstream. People with healthy gut function usually only absorb a small portion of this oxalate (2-3%) and don't need to worry about its toxic effect (unless you eat a very high oxalate diet). People with leaky guts or poor balances of intestinal flora, however, can absorb up to 50% of the oxalate in their food and this can cause all kinds of severe symptoms and conditions. I just wrote a post on my blog listing these symptoms and condition (http://lowoxalateinfo.com/who-benefits-from-a-low-oxalate-diet/). Some of the most common symptoms and conditions are burning pain and itching in the genitals, bladder, rectum, mouth, eyes, joints, muscles and skin, and the formation of kidney stones, but other conditions and their link to oxalate are just as important and not as well known (such as autism, allergies and lichen sclerosis). Your body can also make too much oxalate if you take too much vitamin C, you have a genetic defect, or you have a B6 or B1 vitamin deficiency.
If you are a healthy person with a very healthy gut, you might want to consider the antinutrient properties of oxalate or you might just not want to worry about it. If you started a Paleo diet to heal from a leaky gut, you have poor intestinal flora, or you have any of the symptoms or conditions on my list (http://lowoxalateinfo.com/who-benefits-from-a-low-oxalate-diet/), you may want to minimize or eliminate just the highest oxalate foods or you may want to start a low oxalate diet.
I personally have had 15 of the 26 conditions on my list and I have completely healed from 13 of them during my 20 years on the low oxalate diet and I am making great progress on the last two.
Oh! One thing to be careful about -- the internet is FULL of horrible, inaccurate and out-of-date information about the oxalate content of foods. If you want to explore more about the effects of oxalate on your body here are three websites that provide accurate information:
http://lowoxalateinfo.com/ My blog which also has a Paleo/Weston A. Price slant
http://www.lowoxalate.info/ This site has a short, accurate list of high and low oxalate foods
In addition, if you want a comprehensive accurate list of the oxalate content of foods (about 190 pages), here's a post I wrote about how to get a free one: