When the input of the Schmitt Trigger is LOW, the output is HIGH.
As the input rises, nothing happens to the output until the input is 3v3. This is the voltage on the "+" input due to the effect of the three 10k resistors. These 3 resistors form a voltage divider with two 10k resistors connected to the 5v supply and one 10k resistor connected to 0v.
When the "–" input is 3v3, the output of the OP-AMP goes LOW and it remains LOW until the input falls to less than 1v6. The 1v6 voltage on the "+" input is produced by the three 10k resistors. When the output is LOW, one 10k resistor is connected to the 5v supply and two resistors are connected to the 0v rail. This produces 1v6 on the "+" input.
The purpose of a Schmitt Trigger is to detect and respond to a signal that rises and falls a large amount - in other words it has "large excursions."
There are also signals that rise and fall very slowly - such as a photo transistor detecting daylight.
During the detection process, the output will rise and fall slightly during the morning light and the change from one level to the other will cause the project to turn on and off.
This is unwanted. The Schmitt trigger will produce an output when a definite condition is met and will not change until the daylight is reduced considerably.

PRACTICAL CIRCUITS

Here are some practical circuits using OP-AMPS: