What is backlinks tier?

Nope.

Sorry, but it has nothing to do with that at all!

"First tier backlinks" are backlinks linking to your own site. "Second tier backlinks" are backlinks on other pages linking to the pages on which the first tier backlinks appear. "Third tier backlinks" are backlinks on other pages linking to the pages on which the second tier backlinks appear. The (silly and often incorrect) idea is that you can use spammy/crappy backlinks for "lower tier" backlinks without the risk that your own site will be penalized. (It's often untrue, but people who sell "backlinking services" like their customers to think that it's true, and that those spammy/crappy backlinks will therefore be "safe to buy". It's still silly to buy them. This is all nonsense propagated by people who don't understand that successful SEO, like almost everything else in internet marketing, depends on two things: relevance and quality. But nonsense like this appeals to people who like to look at things in simplistic and quantitative ways, even if those ways aren't really based on anything valid. There are a lot of people like that, though, and of course that's among the reasons why so few people succeed in internet marketing.)

This is the place for SEO questions, guys: Search Engine Optimization