A Tier 1 solar panel is a panel that is made by a manufacturer that has been rated as Tier 1 by a reputable independent PV industry analyst.
Tier 1 is the highest (best) tier, and means that the analyst who ranked it believes that the module manufacturer scores well on lots of criteria including:
- Financial position
- Manufacturing scale
- Deployment scale
- Durability & quality
- Technical Performance
- Vertical integration
- Insurance and backing
- Service and support
I always recommend that non-solar experts buy Tier 1 panels - as it is a simple way to filter out the junk being sold as "Top Quality".
I have written in detail about Tier 1 rankings here.