Mobile-First Indexing is Google’s approach of primarily using the mobile version of content for indexing and ranking.
It matters because technical issues can prevent great content from being discovered, rendered, indexed, or trusted. Strong technical foundations protect rankings during redesigns, migrations, content expansion, and platform changes.
Example
A desktop page may contain a detailed comparison that is missing entirely from its mobile version. Because Google primarily uses mobile content for indexing, responsive changes should preserve important content and links rather than remove them for phone visitors.