The phrase means:
similar in type
There is some discussion of its origins here, noting:
Probably from the meaning of "line" defined as sense #15b in the Oxford Engl. Dict.:
"fig. Plan of construction, of action, or procedure: now chiefly in phr. 'on (such and such) lines.'"
"In all very uncultivated countries . . . there are but obscure lines of any form of government" .
"He had reorganized the constitution on the most strictly conservative lines".
To say that "The plot was something along the lines of a murder and a police man" means that the plot line of the novel was something similar to a murder with a policeman. Or, if you're designing a sculpture and have an idea in mind, you can sketch it out and say you want something along those lines--that is, you want "something like this".