This formula uses -1 for match type to allow an approximate match on values sorted in descending order. The MATCH part of the formula looks like this:
Using the lookup value in cell F4, MATCH finds the first value in B5:B9 that is greater than or equal to the lookup value. If an exact match is found, MATCH returns the relative row number for that match. When no exact match is found, MATCH continues through the values in B5:B9 until a smaller value is found, then it "steps back" and returns the previous row number.
In the example shown, the lookup value is 275, so MATCH returns a row number of 3 to INDEX:
The INDEX function then returns the third value in the range C5:C9, which is $383.