A Schmitt trigger has one input and one output. It sets the output to one of two states depending on the voltage at Vin. Because of the nature of inverting op amp set-ups, the outputted signal will be inverted. When Vin goes below a certain voltage (the lower switching threshold), Vout goes high. When Vin goes above a certain voltage (the higher switching threshold), Vout goes low.
Because Schmitt triggers have an upper and lower switching threshold, they’re much better at cleaning up noisy signals than a single switching threshold. This is because with a single threshold, the noisy signal will jitter between each side of the threshold, causing it to trigger multiple times.