Website Security is the practices and technologies used to protect a website from malware, data loss, spam, unauthorized access, and attacks.
It matters because development decisions affect speed, security, scalability, tracking, SEO, and maintenance. A site that looks good but is fragile or hard to update can quietly hold back every marketing channel.
Example
A site restricts administrator access, removes unused accounts, applies supported software updates, and monitors suspicious logins. These controls address different risks; a security plugin alone does not make the entire site secure.