Risk of ruin
Risk of ruin is the probability of losing a bankroll entirely before any long-run edge can be realised.
In plain English
It depends on stake size relative to bankroll far more than on the quality of selections. A profitable approach staked too aggressively can still go broke, which is why survival is a separate problem from being right.
Example
Risking a quarter of a bankroll per bet makes ruin likely within a handful of losses, whatever the edge.
Why it matters
Being right is worthless if you are not still betting when it pays off.