The '+' signs define the reference direction for positive voltage or potential difference. The quantity Us specifies the magnitude of the source voltage and its algebraic sign with respect to the reference. If Us has a value of +12V, the voltage of node b with respect to node a is positive.
By Kirchoff's voltage law, the algebraic sum Σu of the voltages around a loop at any instant is zero.
Σu = 0 = Uba + Ucb + Udc + Uad
Uba means 'the voltage of b with respect to a'.