At the blockchain level, a post is a root level comment, one that has no "parent".
To users on Hive, this is what is thought of as "long-form content". We can visualize it as an article. Individuals are able to create posts and submit them to the blockchain. The only contingency is if the account has enough Resourced Credits for that base layer activity. If that condition is met, the content is recorded on the blockchain and becomes immutable as soon as the block is irreversible.
Posts on Hive are eligible to share in the reward pool. This is distributed based upon the upvotes received coupled with the voting weight of each vote. It is counterbalanced by subtracting any downvotes received.