This is when someone else uses your email address to send spam. You'll typically receive "bounce back" emails since these emails won't reach the intended recipients.
2ndly this is when someone has gained access to your email account credentials and is using it to send emails. This is a more serious issue.
Boot into Safe mode to run a scan with your antivirus or anti-malware software.
Use a strong, unique password and enable two-factor authentication (2FA) for added security.