This comes from Latin, and literally means 'other things held equal'.
For example, if the price of petrol rose, you would expect more people use public transport, ceteris paribus. However, everything else may not remain equal. e.g. Cost of cars might go down even though cost of petrol went up, etc.
Economists need to use this as a benchmark to understand how two different factors relate to each other. A lot of the time economists speak hypothetically about different subjects in economics. Hope this helps.