This formula is a standard version of INDEX + MATCH with a small twist.
Working from the inside out, MATCH is used find the correct row number for the value in F4, 2100. Without the third argument, match_type, defined, MATCH defaults to approximate match and returns 2.
The small twist is that we add 1 to this result to override the matched result and return 3 as the row number for INDEX.
With level (C5:C9) supplied as the array, and 3 as the row number, INDEX returns "Gold":
=INDEX(level,3) // returns Gold
The above approach works fine for simple lookups. If you want to use MATCH to find the "next largest" match in more traditional way, you can sort the lookup array in descending order, and use MATCH as described on this page.