Overview
An assumption in the context of business or marketing are untested behaviours that might influence if the product actually will be used and if it can be sold towards its customers.
Examples
A product assumes that everyone with an internet connection has a fairly new device with at least 16GB RAM.
This can be risky because if the product is aimed towards a large group of customers, many of them might not have the latest and greatest computer hardware. If the application that is being developed can only run on said hardware, many users have a serious problem