In more detail
Google makes changes to Search constantly, most of them small. Several times a year it releases larger, announced changes to its core ranking systems. Google's core updates documentation explains that these are designed to improve how its systems assess content overall, and that they can take a couple of weeks to roll out fully.
Google also runs separate, named updates for specific systems, such as spam updates. Its ranking systems guide describes many of the systems involved, including those for helpful content, reviews, spam and freshness.
After a core update, rankings can move noticeably even when nothing on a site has changed, because other pages are now being assessed differently. Google advises site owners who see drops not to rush into changes, and to review their content against its guidance on helpful, reliable, people-first content. Recovery, where it happens, can take time and may not come until a later update.
Why it matters when hiring an agency
Core updates are one reason no agency can guarantee rankings. Your position can change because of how Google reassesses the whole web, not just your site.
When interviewing agencies, ask how they handled a past core update for a client: what they checked, what they changed and how quickly. A good answer involves calm diagnosis, comparing affected pages with competitors that gained, and improving genuine quality, not chasing quick fixes. Also ask how they'll tell you whether a traffic change is due to an update, seasonality, a technical problem or their own work. Separating these is a sign of real experience.