Plain English Meaning
Do what you decided to do, then check that it actually worked. If it didn't, change what you're doing.
Full Definition
In the Three-P Model (Perceive–Process–Perform) used for aeronautical decision-making, Perform is the action stage in which the pilot carries out the chosen course of action, then monitors the outcome to confirm the situation is improving and adjusts if it is not.
Context Anchor
Seen in aeronautical decision-making discussions, especially the Three-P Model: Perceive, Process, Perform.
Derivation
From the Latin 'per-' (through) plus 'fornire' (to provide or accomplish) — to carry through to completion. In the Three-P Model, it specifically means carrying the decision through into action and watching the result.