In the diet, oxalates block absorption of calcium by bonding with the calcium. How much it blocks is affected by many variables. Heat seems to increase the effect, and vitamin A seems to decrease it. You can experience the basic effect by drinking milk after eating rhubarb. The grittiness you taste is the calcium bonding to the oxalates.
Oxalates can be fatal to humans in high enough doses in plants. Many plants with milky sap or gritty leaves (like rhubarb) are high in oxalates. Many species of tortoises in the wild eat plants so high in oxalates they are considered toxic to humans without visible harm- however, wild torts also seem to frequently have stones.
Oxalate levels in a plant vary- they can be high in stems or leaves and absent in flowers or roots. This is one reason some parts of rhubarb are edible and some are not.