“To spoof” means to trick or deceive and is an apt description. Criminals send emails with the intent to deceive recipients with phishing or blackmail schemes.
This is done, for example, by sending emails from fake sender addresses which may look like trusted and known sender addresses. Cyber criminals register domains for this purpose which are very similar to the imitated sender addresses and, at first glance, do not raise any suspicions.
Another method used by hackers is the manipulation of email headers. Here, the Sender field is changed so that it is hard, or impossible, to distinguish it from the original.
This allows the sender to send messages that appear to come from a known or trustworthy source, in some cases even from your own account.