一本 (first tier) universities admit in the first round; 二本 in the second; 专科 (specialist colleges) later still. Tier determines admission timing more than it determines academic quality.
In practice, the top-ranked national universities all admit as 一本, and the correlation between tier and reputation is strong at the top. But there are excellent programs at nominally 二本 schools, particularly in engineering and applied fields.
The new gaokao provinces (新高考) have been gradually merging 一本 and 二本 into a single batch called 本科批, replacing 'tier' with a smoother continuous admission process. Check which system your province uses.
The 双一流 (double first class) designation, introduced in 2017, marks a set of universities and disciplines the state has selected for elite funding. It is a more current signal of national investment than the old 985/211 lists.
For most students, the useful shorthand is: which universities have your target major, and where do their historical cutoffs fall? Tier is a starting filter, not the answer.