Hiya Christine!
I'm new too, but I can help a bit with the physiology.
Kidneys 'filter' your blood. Basically, a kidney is a cluster of tiny tubes (tubules). When blood passes through the kidney, most of the blood passes into these tubules and the 'useful' bits are then reabsorbed back into the body and continue to go round in the blood. The bits that the body doesn't think are useful remain in the tubes - i.e. they don't get re-absorbed - and the tubules eventually lead to the ureters, the tubes which go to the bladder, and from there this 'waste' product goes out of the body (we call it 'urine' like one of the waste products that doesn't get reabsorbed, 'urea').
The important bit here is that 'most of the blood passes into the tubes'. Not ALL of the ingredients of blood pass into these tubes and one of the ingredients that doesn't (as far as I understand it) is a certain protein (lets call it X). Protein X is too big to pass into the tubules, so it never leaves the blood.
If you find protein X in your urine, then logically it must have come from the kidney. But it isn't normal for protein to pass into the tubules. Therefore, it must have 'leaked'.
Doctors monitor our urine for protein regularly, so that if the kidney is 'leaking,' they can find out very quickly and treat / monitor as necessary. What has your doctor advised?
There are other reasons for finding protein (other proteins, not necessarily protein X) in your urine and not all are related to leaky kidneys, but if your doctor has said that it is down to that, they would have ruled the other things out.