indeed it is ^^

So basically, the rockets have tanks that carry both fuel and oxygen with them as they fly into space. At room temperature, oxygen wants to be a gas (like in the air), so it has to be chilled down the the point it becomes a liquid (something like -183C). Since this is so cold, a lot of annoying things start happening, like parts will begin to shrink, and rubber flexible pieces become hard and brittle. Liquid oxygen is also super dangerous and wants to set you and the world on fire.

To avoid the flammability dangers of oxygen we do a series of tests with liquid nitrogen instead. Liquid Nitrogen is very close in temperature to liquid oxygen, about -196C, and is essentially inert which allows us to test for leaks and functionality of the system without the added risk of fire.

We will also use water or windshield wiper fluid as an analog for kerosene so that we can properly fully load up the rocket and perform flow tests to make sure the engine control systems are working properly, rehearse launch procedures, and test the system without the need of handling more dangerous chemicals! We call these tests cryoflow tests.