What "appropriate source of randomness" in php random_int means?

Question

I was writing some PHP code when I came to an warning in my coding saying "Unhandled Exception" pointing to the use of random_int in some part of my algorithm. I wasn't aware that random_int could throw an exception so I made a research and the documentation said that the exception occurs when "(...) an appropriate source of randomness cannot be found(...)".

My question is: what does exactly "appropriate source of randomness" is? I tried to google it but I didn't find any answers.

If this is to much of an open-ended question, please, consider only the scope of the random_int function.

To provide more information, I'm using PHP 7.4.3 (NTS) in Ubuntu 20.04.4 LTS. The message that I'm receiving is "Unhandled exception" and nothing else but the line and column where the exception is unhandled.