In this case, ultrafiltration refers to a function of the kidneys in the human body. Specifically, it is the process in which fluid passes through a semipermeable membrane. This means that some particles are allowed through the membrane but not others. When the kidneys perform this function, it is called glomerular filtration and is the first step to making urine.
Glomerular filtration occurs when blood passes through the glomerulus. The glomerulus is the body’s filter. It allows some smaller molecules, such as water, to pass through. This watery filtrate, which includes many beneficial materials for the body (such as amino acids, glucose, and potassium), is then reabsorbed into the body later on in the process. Everything else, such as larger molecules and other toxicants (for example, drug residue) is caught in the glomerulus and transferred from the blood into the urine. The ultimate filtrate, then, is this remaining collection of substances that was caught in the filter of the glomerulus. It will go on through the process of becoming urine.
To review, the process of ultrafiltration is what the body does in order to filter (or "clean") the blood and create urine, which contains waste that the body does not need. This takes place in the kidneys, where the glomerulus (a semipermeable membrane filter) is located.