On an L-head engine the inteke and exhaust valves are located in the engine block instead of the cylinder head. On motorcycles the valves are located in the cylinder itself instead of the cylinder head.
On the OHV engines the valves are in the cylinder head and hanging over the cylinder bore instead of being located beside it.
L-heads are also called sidevalves, flatheads, or even Ricardo heads after the guy who designed it originally.
The L-head design does not flow as well as the OHV designs but are able to put out some very good low end torque and they’re much, much simpler to work on.
Hope that explains it.