A base rate is the underlying frequency: how often breakouts follow through, how often a given pattern resolves up, what share of day traders are still funded after two years. Neglecting it means judging a case entirely on its own colourful details.
You see it whenever someone argues from an example. A breakout looks clean, so the fact that most breakouts in a chopping market fail gets no weight. A prop firm advertises payouts, so the share of accounts that never reach a payout never enters the conversation.
Base rates are boring and they are the strongest single input you have. Find the number first, then let the specifics move you off it a little, not a lot.
Related: representativeness-heuristic, survivorship-bias-in-advice, sample-size, availability-heuristic