(I had never heard of it before you asked, but I’ll try to answer anyway )
From wikipedia (a surprisingly good site for math definitions):
“Itō calculus, named after Kiyoshi Itō, treats mathematical operations on stochastic processes. Its most important concept is the Itō stochastic integral.”
So that may not be very helpful, but follow me a moment. What is meant by the word “Calculus” in Mathematics? The same as in ordinary language, that is, a system or method of calculation. I could say that long division is a calculus. People say that utilitarians perform an “ethical calculus” to make decisions. The college courses usually called calculus refer to the calculus of Reimann integrals, that is, how to do calculations on and with them.
Now, from what I can figure from the wikipedia page, Itō calculus is a system of calculation for stochastic processes. What are stochastic processes?
Again from wikipedia:
"In the mathematics of probability, a stochastic process or random process is a process that can be described by a probability distribution.
The two most common types of stochastic processes are the time series, which has a time interval domain, and the random field, which has a domain over a region of space.
Familiar examples of processes modeled as stochastic time series include stock market and exchange rate fluctuations, signals such as speech, audio and video - medical data such as a patient’s EKG, EEG, blood pressure or temperature; and random movement such as Brownian motion or random walks. Examples of random fields include static images, random terrain (landscapes), or composition variations of an inhomogeneous material."
This seems to answer the “what is it used for”, no?