Initial risk is fixed at entry; open trade risk is dynamic. Buy at $50 with a stop at $48 and open risk is $2 per share. Move the stop to $50 after a rally and open risk is zero. Move it to $52 and the position has locked in profit - open risk is negative.
Tracking it per position is what lets you run several trades without exceeding max-open-risk. A winner whose stop has been raised is no longer consuming the risk budget, so a new idea can be funded without increasing total heat.
The caveat is that "zero risk" assumes the stop fills. Overnight and in fast markets it may not, so a breakeven stop is better described as risk reduced rather than risk removed. See gap-risk and stop-slippage.
Related: initial-risk, max-open-risk, risk-free-position