What is a 'dynamic entity' in computer science?
There are no scientific term "dynamic entity", so meaning of this pair of words can be well stretched. In this particular case they could write something like "A computing system ... is a dynamic system", but apparently they didn't like the repetition, so they have decided to use a fuzzy word "entity". Also, there is a mathematical term Dynamical System, which doesn't have anything in common with what they are trying to say.

Essentially they mean that there is a lot of parts in a real computing system, which can change with time - hardware and software can be installed and uninstalled, software can be configured in many different ways, data comes into the system and gets out of it and so on. Sometimes even users and people, supporting a complex computing system, can be considered as a part of it.