Spam is the electronic equivalent of junk mail, unwanted emails such as subscriptions or promotional offers. Although an inbox-clogging nuisance, it is not as dangerous as phishing. It can, however, be used as a technique for hackers to fill up your inbox so that you miss important security alerts.
How to:
In Gmail, select or open an email
In the top right, tap the three dots and then click “Report spam”.
The email is then removed from your inbox and sent to the “Spam” label. Google will receive a copy of the email and may analyze it to help protect other users from spam and abuse.
Here are some things to look out for / common reasons to mark messages as spam:
The email address has been spoofed (impersonating a legitimate user)
Phishing scam
Messages from an unconfirmed sender
You tried to unsubscribe from this sender
Messages content is empty