The backspring is the flat steel spring running along the back of a slip-joint knife that presses on the blade tang. It holds the blade open, holds it closed, and produces the snap that traditional knife buyers listen for.
A slip joint has no lock. On one built with a backspring, the spring bears on the flat of the tang when the blade is open, which resists closing, and on a different flat when the blade is shut, which resists opening. That is the entire retention system.
On a multi-blade pattern like a stockman, one spring can serve two blades, or the knife can carry two or three springs. That is part of why traditional patterns are described by both blade count and spring count.
A stronger spring gives a firmer snap and a harder pull. A lighter one is easier on fingernails. Neither is better; makers tune it deliberately for the pattern.
Springs weaken very slowly over decades. A flush, level spring with the blade open, closed and at the half-stop is a mark of good fit and finish.