A near miss is a losing spin that looks as though it nearly won: two bonus symbols with the third just above the payline, for example. It is still a loss, and it was no closer to winning than any other losing spin.
On an RNG slot the result is decided before the reels stop. The third symbol did not "just miss"; the outcome selected was a non-winning one that happens to be drawn that way. Weighted reels and virtual reel mapping can make high-value symbols appear next to the line more often than they land on it.
Near misses matter because they can feel encouraging and make it harder to stop. Recognising them for what they are, an ordinary loss with a particular look, is one of the most useful things to know about how slots present results. Nothing about a near miss makes the next spin more likely to win.
See our safer gambling page for more on how game design affects play.