Reach tells you how far a post travelled. Ten people who each saw a post twice give a reach of 10 and 20 impressions. That is why reach is never higher than impressions.
Organic reach comes from followers, shares and recommendations, paid reach from ads. Platforms count it in their own way and over their own periods, so compare reach within one platform rather than across them.
Example
A café's post is shown to 1,200 different people, and 300 of them see it twice. Its reach is 1,200 and its impressions 1,500.