An industrial-strength thing is very strong — first of all, strong enough for factories and heavy work, not for ordinary home use. Industrial-strength cleaner, glue, or coffee is the fierce version, made for a job that would defeat the supermarket bottle. Though actually, people now use it as a joke for anything that feels too much: industrial-strength perfume, industrial-strength headache pills, even an industrial-strength sulk. In that stretched use it is informal and humorous; you are not really talking about a factory. Used of a real product, it is more literal and a bit of advertising talk. Either way, the idea is the same: stronger than you expected, sometimes stronger than you wanted.