Entropy is basically the measure of unpredictability.
In other words, "how random is your random"?
In order for any Data Security system to work, it requires the input of random numbers.
They need to be unpredictable to a potential bad actor or else it does not matter "how many bits are in your encryption" (key sizes/algorithm strength).
Computers are inherintly deterministic - they do what software instructs them to do, and thus are predictable which is the source of countless security incidents.
Entropy-As-A-Service (EaaS), in general, refers to client devices obtaining entropy over the Internet from service providers with special hardware entropy sources.