Welcome to the wonderful world of public key cryptography.  Your Nostr account is made of a mathematically related pair of very big numbers called a public key and a private key.  You public key is publicly available and peole can use it to verify that messages and posts you send actually came from you.  They can also use it to send you private messages.  Your private key is used to sign messages and read private messages.  Your private key is similar to a username/password.  It effectively is your account.  It can be used with any Nostr app to access your account, but make sure the app is trustworthy.  Keep your private key secret and back it up. 

 Thanks a lot.