86% of the worldwide email traffic is basically... spam. In a response to that, spam filters and anti-spam organizations have become numerous and sharp. Which is why, as a cold emailer, you might have a tough time reaching your prospect's inbox 100% of the time.
But it doesn't mean it's not possible or -at the very least- that you can't reach most of them. You just need to know how to overcome the hurdles of the spam filters.
In other words, deliverability is your likeliness to reach your prospect's mailbox.