An impression is counted every time your content is rendered on a screen, whether or not it is the same person seeing it again. Because of that, impressions are always equal to or greater than reach. The ratio between the two — impressions divided by reach — is your average frequency, or how many times the typical viewer saw the post.
Impressions matter for understanding exposure and, in paid media, for pacing and cost. A rising impression count with flat reach usually means the same people are seeing a post repeatedly, which can signal strong resonance or, in advertising, creative fatigue and wasted spend.
Use impressions alongside reach rather than on their own. For organic content, reach tells you how many people you touched; impressions and frequency tell you how hard the algorithm is pushing the post to the people it already found.