Tier #1 - Are the backlinks that point directly to your website.
Tier #2 - Are the backlinks that point to the Tier #1 backlinks.
Tier #3 - Are the backlinks that point to the Tier #2 backlinks.
and so forth...
Most probably you heard that from some gray or black hat optimizers. Tier linking is not as effecting as it used to be, so stop wasting your time and get some real quality content on your sites instead.
Hope that helps!