"s.t." means to obey, obey. ...

For example:

Objective function: min {x+2}

Constraint: s.t. x={ 1, 2,3}

When the minimum value of x+2 is found, the value of x is 1, 2, 3.

Or when the value of x is 1, 2, 3, find the minimum value of x+2.