[TL;DR:] The key distinction to make, I think, is between the next event's theoretical probability v/s the cumulative empirical probability.
The Gambler's Fallacy of assuming the probability of the 10th toss being anything but exactly 50/50 is wrong (assuming the coin is fair).
However, since the probability should be 50/50, you are most likely to get 45 heads and 45 tails over the next 90 throws. So if the proportion of heads was 90% in the first 10 tosses, it will be ~54% over 100 tosses (including the ten before) — regression (moving closer) to the mean (here, 50%).
[Update:] Hadn't noticed it before, but @AndreNicholas got to it before me in the comments.