Phytic acid is a plant's method of storing phosphorus. It's indigestible by humans as we lack the enzyme, phytase, that degrades it (these enzymes are typically found in microorganisms, so some gut microbes may be capable of breaking it down for us).

The problem with phytates (salts of phytic acid) is that it is theorized that large amounts of it can cause mineral deficiencies. Phytates are potent chelators, which means they go after desirable metals that our bodies utilize (such as iron, zinc, copper, etc). Once bound, these minerals are not bioavailable to us, as mentioned above, we lack phytase that is capable of breaking it down.

Seeds/grains and nuts are high sources of phytates. Seeds (think wheat) aren't consumed in high amounts by paleo folks, so that already eliminates large amounts of phytates from our diets.

Phytates may have been unfairly vilified however. They're listed in the laundry list of reasons why not to eat grains such as wheat. However, I've not heard of anybody with severe deficiency caused by excess phytate consumption. In fact, phytates may be somewhat beneficial as natural chelators. Removing things like toxic, undesirable heavy metals as well as possibly mitigating high iron load caused by large amounts of animal protein consumption (fairly typical amongst paleo folks).