Email spoofing is the creation of email messages where the sender email is not the owner of the mailbox (ie the sender email has just been forged).
Because the core email protocols do not have any mechanism for authentication, you can send an email from every email address you want.
spam and phishing emails use it to mislead or even prank the recipient about the origin of the message.
spoofing is central to most phishing attacks.
To prevent spoofing, you configure the email authentication mechanisms (SPF and DKIM)