What it means
Subtitle synchronization is the relationship between caption display times and the associated audio or scene. You inspect it against the actual media, not just against other timestamp numbers.
Constant delay versus drift
If a known line appears 0.75 seconds too early at both the start and the end, a uniform positive offset is a reasonable correction to test. If the required offset changes as the recording progresses, one shift is not enough. An edited recording or a different delivery version may require new cue timing.
Work through an example
Suppose a line should begin at 12.000 seconds but its caption begins at 11.250. The difference is 12.000 − 11.250 = +0.750 seconds. In the time shifter, positive delays the whole file and negative moves it earlier.
Check several lines after the change, including one near the end. Preserve your input. The shifter stops if a cue would fall before zero, exceed its time limit or require resolving an existing overlap; it does not discard inconvenient cues.
What this cannot establish
A uniform shift does not discover word boundaries, verify a transcript or repair every local edit. Review cue text and reading speed separately with the checker.