The exact spreadsheet formula for the thing you're trying to do

You know what you want the spreadsheet to do but not the formula that does it. Describe the outcome and where the data sits, and this gives the exact Excel or Google Sheets formula, a one-line explanation of how it works, and the one gotcha that trips people up. No trawling forum threads.

I want [described outcome] in column [X], based on data in [Y]. Give me the Excel or Google Sheets formula, plus a one-line explanation of how it works and one common gotcha to watch for.