Albumin is a protein in your body, produced by your liver. It makes up about 60% of the protein in your blood. It has many important functions including building muscle, repairing tissue, and fighting infection. It also acts a bit like a sponge to hold fluid in your bloodstream instead of it leaking into places it shouldn't be.
Albumin and other proteins aren't usually found in urine (pee), or they're only found in tiny amounts, because healthy kidneys stop most of your protein from entering your urine. If your kidneys are damaged, larger amounts of albumin and other proteins can pass through their filters and into your urine. If other proteins (as well as albumin) are found in your pee, it's usually called proteinuria.
Having albumin in your urine (called albuminuria) is not a separate disease, but may be a symptom of many conditions that can cause kidney damage.