Followers’ reachability is an index showing the chance of your followers seeing your posts. So:
Easy to reach — those followers who see your content each time you post it;
Unreachable — don’t see your posts at all. This can happen when those Instagram users follow a lot of accounts larger than yours, don’t check their news feed or use the app at all, or they’are just bots.
Good Instagram account has more easy reachable followers than unreachable ones, because having tons of hard- or unreachable followers can decrease engagement rate on the profile, since those users do not interact with the content of the Instagram account.