A pier is the directory which contains the state of an Urbit ship. It is automatically created in the current directory when booting a ship for the first time.
Your pier should be kept safe - if it's deleted, you will need to perform a breach in order to advise the rest of the network that your ship's state has been lost.
Note that a backup should usually not be taken of your pier as a means of keeping it safe - once the ship is run on the network, the backup will be outdated and running from it will necessitate a breach. A running ship automatically backs up its own state within the pier directory for recovery purposes, so as long as the pier is preserved, recovery from most problems is possible.
Event Log: The main important content of the pier directory.
A Guide to Breaches: Instructions on handling continuity breaches (resets) on the Ames network.
Ship Troubleshooting: General instructions on getting your ship to work, which includes network connectivity issues.