What is the significance of calcium oxalate crystals in the urine?

Usually nothing.  Calcium oxalate crystals are a reasonably frequent finding on urine microscopy.  The crystals are sometimes described as looking like "envelopes", but judge for yourself.  In and amongst the scattered cells depicted below are the distinct squares-with-crosses (or perhaps "pyramids"?) that are the calcium oxalate crystals.

Such a finding is usually benign, as these can be normal.  However, they are present in increased amounts in at least one noteworthy medical condition: ethylene glycol poisoning.  Since ethylene glycol is mostly found in car "antifreeze" and in brake fluid, poisoning usually occurs if these substances have been ingested in significant amounts.  (The whole process is helped along, especially in children, but the substance's sweet, innocuous taste.)

Incidentally, ethylene glycol is one of the differentials for an increased anion gap metabolic acidosis - see here.