In one sentence
OG tags (Open Graph Protocol) are meta tags that control the preview rendering (title / description / image) when a URL is shared on Twitter / Facebook / LinkedIn / Slack and similar platforms.
What does it look like in practice?
For example, suppose someone tweets https://geo-meters.com/learning on Twitter.
If OG tags are set, you get:
┌─────────────────────────────────┐
│ [Card image] │
│ │
│ Learning | GEO Meter │
│ The frontline of the AI search… │
│ │
│ geo-meters.com │
└─────────────────────────────────┘