This comes from the solver's presolve.
It means that the model really is infeasible and that this can be determined before the numerical solution algorithm really starts. The presolve finds that one of your equations has just one variable in it and so can use that equation to determine the value of that variable. Once that variable's value is fixed, there may be other equations that now involve just one variable – and so on, that's the pre-triangular part of the model.
If it turns out that after fixing some of the variables there is now an equation with no variables left, and the equation is not satisfied, then the model is infeasible.
It might also have an equation with one variable left but the equation now says that variable must have a value outside its bounds. That also means the model is infeasible.