Small daily fluctuations of a few positions are normal and rarely mean anything, while a sustained directional drop or a large sudden shift across many pages at once is the kind of movement worth taking seriously. The line to hold is small-and-noisy versus sustained-or-broad. Treat any figures here as a working guide rather than a fixed threshold, because what counts as normal depends heavily on the term and the page.
Routine noise is the default state of search results. A page may sit at position six one day, eight the next, and back to five the day after, and that wobble usually reflects personalization, location, device, ongoing tests, and the constant small re-shuffling Google does as it weighs signals. Newer pages and competitive terms swing more than established pages on settled queries, so a few positions of daily drift on a hard keyword is closer to expected behavior than a problem. Tracking a single day in isolation almost always overstates how much is really happening.
The pattern, not the day, is what carries the signal. A drop that continues in the same direction over a week or two, rather than bouncing back, suggests something real has changed: a competitor improved, intent shifted, or a quality or technical issue has surfaced. A sudden large move that hits many pages at once, not just one, points to something site-wide such as an algorithm update, a crawling or indexing fault, or a recent change you made. Those two profiles, sustained and broad, are the ones that justify investigation, while isolated daily jitter on one term rarely does.
The practical move is to stop reacting to the daily graph and watch the trend instead. Set a baseline over a week or two for your important terms, ignore the small back-and-forth that lands within that normal band, and only dig in when you see a drop that holds its direction or a shift that spreads across many pages. When you do investigate, check recent changes, the SERP, and any update timing before assuming the worst, since most movement resolves itself on its own.