SUMIF adds up the cells that meet one condition:
=SUMIF(range, criteria, [sum_range])
range is where Excel looks for the condition, for example the Region column.criteria is what to match: "West", ">1000" or a cell like G1, with the same rules as COUNTIF.sum_range is what to add, for example the Amount column.
Make range and sum_range the same size and start on the same row. If they don't, Excel resizes the sum range on its own, which can add the wrong cells without any warning.
When the condition is about the numbers you're adding, leave sum_range out: =SUMIF(C2:C12,">1000") adds every value over 1,000.