A block is when the email server you are sending to refuses your email. There can be lots of reasons for this. They might decide you are spam, or that you violate an internal policy they have, or the user you are sending to may no longer exist on the mail server.

A bounce is an email that will be sent back to you because of some error. Bounces can be categorized as hard-bounces, or soft-bounces. A hard bounce will be sent back because of an invalid address. A soft-bounce will reach the recipients email server, and then be sent back due to a full destination inbox, a server error, or an email address that no longer exists. Here is a list of soft and hard bounce codes and reasons: https://support.activedemand.com/support/solutions/articles/12000012719-do-you-have-a-list-of-bounce-codes-

A drop is if it has already bounced, or if it is invalid. These occur to help protect sender reputation.