What are cryptographically secure rand() values?

Question

PHP

rand() - Generates a random number
Warning: This function does not generate cryptographically secure values ​​and should not be used for cryptographic purposes.
What is the difference between cryptographically secure and insecure? What are they actually talking about here?

Answer 1

The randomness of the numbers generated by this function is not random enough to be used for cryptographic purposes. In a certain way, you can predict which number will fall at a certain moment, or even predict a whole sequence.