Malware is harmful software — the word is short for “malicious software”. On a website it is unwanted code that someone sneaks in to damage the site, steal information, or quietly use it for their own purposes, such as sending spam or attacking other sites.
Think of it like an infection: it slips in through a weak spot, spreads, and makes the site behave in ways the owner never intended — showing strange pages, redirecting visitors, or slowing everything down. Often the owner does not notice at first, which is what makes it dangerous.
Protection comes from keeping everything up to date, using strong locks like an SSL certificate and a Firewall, and always having a fresh Backup so a clean version can be restored if the site is ever infected.