What dc load line?

well, this is very simple to understand ,

the DC load line of a circuit is nothing but the Kirchoff's voltage law on the out put circuit of the transistor-amplifier. As the KVL is linear equation involving voltage drops the equation of load line is a straight line

let us assume that the transistor is in CE configuration.

VCC-VCE-ICRC=0v

ICRC=VCC-VCE of the form by=aX+c which is in the form of a straight line with positive intercepts on X(VCC) and Y(VCC / RC) axis and a negative slope

where,

x= VCE/RC

y= IC

m(slope)= -VCC/RC