what and how/
Seeds that we use in our diet (most common ones being nuts, grains, seeds of pseudo-cereals and legumes) need to be properly soaked before consumption, to deactivate phytic acid in them.
Soaking should be done in slightly acidic water above body temperature. For achieving the acidity, a few drops of lemon juice or vinegar should be added to the water. 
Usual soaking time for most of the nuts is around 8 hours, while soaking time for most of the cereals is 2-4 hours (if you want to consume them raw, it's good to soak them for 8 hours as well, or, even better, to sprout them) and for legume seeds it is usually 8-10 hours. After soaking the water should be discarded and the seeds should be rinsed with clean water.

Phytic acid is a chemical compound that the plants use as a storage form for phosphorous. It is found in greater quantities in the seeds. In human digestive system, phytic acid forms complexes with minerals such as calcium, zinc, magnesium and iron, inhibiting their absorption in human body. That way our body discards the precious minerals rather than making them its building material. For that reason, phytic acid can be called an ''antinutrient''. By soaking the seeds, we activate enzymes called phytases, which are naturally responsible for decomposition of phytic acid. 

what if/
I will not write much about soaking grains and legumes as I don't normally use them in my diet (ok, I will write about buckwheat - a pseudocereal - in one of my next posts). But here is one interesting and useful information - oats don't have enough of their own phytases to decompose all their phytic acid, and that's why it is useful to add a little bit of some other grains (buckwheat, wheat, spelt, kalmut) to soak with them, so that their phytases could neutralize phytic acid from oats.

This was just a very brief talk about soaking seeds. If you want to know more details, there is plenty of scholarly articles you can find on the internet. Explore!