Phishing is Internet Public Enemy Number 1.

In that it is how the majority of computer and smartphone virus infections and malware are spread.

It normally relates to inbound emails that attempt to impersonate a source trusted by the recipient, maybe PayPal, a bank, telecom provider, etc, involving some sort of invoice or refund, and tricking the recipient into clicking on a link that may install malware, steal confidential login details, etc.