If you want to digitally sign messages or enable others to send you encrypted messages, you must have your own personal public/private PGP keypair.
Your public key should be made available to any person that you want to be able to send you encrypted messages that only YOU can decrypt (using your private key). Additionally, your public key needs to be made available to anybody you send a digitally signed message to - the public key is required to verify the digital signature.
Your private key is necessary to decrypt any message encrypted using your public key. Additionally, your private key is needed if you want to digitally sign a message. Your private key will be protected via a passphrase created when the keys are generated. Thus, to perform any activity that requires your private key, IMP will prompt you for your passphrase. You only need to insert your passphrase once per session - IMP will store your passphrase until you logout (So be sure to logout!).