Email spoofing is when someone sends an email pretending to be someone they are not. Such email appears to be originated from a trusted sender - such as customers, vendors, coworkers or managers, or even yourself - but they are actually sent from cyber-criminals, who would trick you into transferring money to them, downloading an attachment with malware that could infect to your entire network, providing them your personal information for fraudulent purposes, and more.
The following is an example of a spoof mail.
From this sample, we can see that the mail was actually sent from email@example.com, not Eric Tan. Although the mail content looks legitimate, with professional-looking email signature, we should always be careful and double check on the sender email address before we make any response to the sender.
For more information on how to identify a spam mail, please refer to this article: How to identify whether an email is a spam mail?