A plan turns trading from a series of decisions under stress into execution of decisions made calmly. It covers markets and sessions, setup criteria, risk-per-trade, daily loss limits, position-sizing, and review routines.
A plan you do not follow is a wish list. The trading-journal measures plan adherence separately from profit, because you can follow the plan and lose or break it and win.
Example: a plan states: ES only, 9:30 to 11:30 a.m., two setups, 0.5% risk, stop for the day at -1.5%, no trades within 5 minutes of tier-1 news. Every trade is graded against it.
Related: trading-journal, risk-per-trade, process-over-outcome, overtrading