Definition
Brake-specific fuel consumption (BSFC) is the mass of fuel an engine uses per unit of power produced per unit of time, allowing engines of different sizes to be compared on efficiency independently of their output.
Litres per hour tells you what an engine is burning. BSFC tells you how efficiently it converts that fuel into work, which is the only fair way to compare a small auxiliary with a large inboard.
Reading a BSFC curve
BSFC is not constant. Every engine has a region of its load and speed range where it is most efficient, and it gets worse at very light loads and at the top of its range. That curve is the technical reason behind the advice to cruise at a sensible load rather than at idle or flat out — see best rpm for a marine diesel.
The limit of the number
BSFC describes the engine, not the boat. A hull that is fouled or a propeller that overloads the engine will ruin real-world consumption regardless of how efficient the engine is on a test bed. That is why the number worth planning with is the one you measure yourself — see how to measure fuel consumption.
Where you meet it
In manufacturer data sheets and repower comparisons, usually in grams per kilowatt-hour.