There may be external circumstances or events that must occur for the project to be successful. If you believe these external events are likely to happen, then you have an assumption. If an event is within the control of the project team, such as having testing completed by a certain date, then it is not an assumption. If an event has a 100 percent chance of occurring, then it is not an assumption, since there is no "likelihood" or risk involved (it is just a fact).
Examples of assumptions might be "Budgets and resources will be available when needed," or "The new software release will be available for use by the time the development phase begins."
I've also put together a short online course that describes project management fundamentals if you're interested in learning more about this profession.