why and when does a warning "Minimum step size reached; singularity possible." appear when using the function quad?

why and when does a warning "Minimum step size reached; singularity possible." appear when using the function quad?
can anybody explain the process? how does quad reach a designated tolerance? thank you very much.

[Lin LI]

ok ,thank you. I have removed the message.

[Jan]

Then I've removed my former comment also.