Role or job creep is a situation where job responsibilities keep piling up.
Role creep is especially bad in software development as the demand for developers is much higher than their availability. It could be said that the roles such as the full-stack developer or devops engineer are born out of this.
As a developer, I have experienced role creep many times. The reason for this has always been either leaving colleagues or the lack of resources to hire more professionals to do certain tasks.
Role creep can go in multiple dimensions. You can take on different types of tasks, such as backend tasks and frontend tasks. But you could also take on same types of tasks in different projects or product lines. In both cases you should consider whether the additional work load is actually doable. You should also negotiate for a fair raise from increased responsibility although raise would not help while burning out under the burden of work.
In freelancing, a role creep can also happen when you get hired to build something but in addition you assume the responsibility of operating it. You might not even realize it at first. Building something is very different to operating it. The result is that you do not get fairly compensated, and when a server blows up, you become responsible. This is one of the biggest mistakes I made in freelancing.