Initial risk is entry price minus stop price, times size. It is set once, before the trade begins, and it is the denominator for that trade's r-multiple forever, even if the stop later moves.
Recording it correctly is the single most important data hygiene rule in a trading-journal. If you re-baseline R when you move a stop, your statistics become incomparable and flattering: every trade where you tightened a stop will show an inflated multiple.
Example: long 200 shares at $30.00 with a stop at $28.50 means initial risk of $300. Exiting at $33.75 is a $750 gain, or +2.5R. If you had trailed the stop to $32 first, it is still +2.5R, not +1.2R against the trailed risk.
Related: open-trade-risk, r-multiple, risk-normalisation