A checkpoint captures the state needed by a particular system, such as progress, intermediate results or an execution environment. Its contents and consistency requirements depend on what must be resumed.
Recovery needs a valid checkpoint compatible with the current execution and its external state. Restoring internal progress does not automatically undo actions already performed outside the saved environment.
ELI5
Think of saving a game's progress so you can continue from a known point rather than starting again after an interruption.
For example, a long-running experiment can retain its completed work and necessary state before the next stage, then resume if the process stops.
Is every saved file a complete checkpoint?
No. It must include the state required for the particular recovery or continuation being claimed.
Does restoring a checkpoint reverse external actions?
Not automatically. Messages, payments or other external effects need separate reconciliation to avoid unsafe repetition.
