A summary, not an outcome
A polling average is a summary of selected surveys. Its meaning depends on which polls qualify and how the calculation treats time, population, and repeated polling from the same organization.
Ask what went into it
Before reading a change as a shift in opinion, inspect the included surveys. A new poll or an expired observation can move an average even if no pollster has measured a comparable change.
A lead is not a probability
A candidate’s margin in an average is not the chance that candidate will win. A forecast needs additional assumptions about uncertainty and the electoral process. Neither product removes the need to inspect the underlying evidence.