(noun)
a turning wheel that has teeth around its edge into which a small piece (a pawl) fits in order to ensure that the wheel turns only in one direction, or some mechanism that uses such a setup;
e.g. This machine contains a ratchet to ensure that it does not malfunction.
(noun)
a steady increase or decrease in the amount of something;
e.g. There has been a downward ratchet in the price of houses in the area.
(adjective)
in informal usage, low-class or characteristic of lower classes in terms of being unrefined or overly flashy, or resembling this impression;
e.g. They wore ratchet clothes made of cheap material.