I Just Took My 3 Year Old Daughter To The

I just took my 3 year old daughter to the doctor. She is having trouble with peeing in her pants just drizzling in them and it hurts when she pees. They did a urine test and it showed no UTI but they are going to let it sit over night to see if anything shows. They told me that she did have some crystals in her urine and that indicated not wiping properly and from holding her pee way too long. Please help me understand this a little better... what does crystals in a kids urine mean?