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The curly braces will surround the entry in a cell when the formula is entered as an Array (with Ctrl+Shft+Enter) or when there as been a Data Table created (which can be found on the ribbon under Data | Forecast | What-if analysis | Data Table). When the Data Table has been created, cells involved cannot be edited piecemeal.
Check out this and other videos on youtube in the ExcelIsFun channel and Bill Jelen's channel to discover a bit about how Data Tables work. https://www.youtube.com/user/ExcelIsFun/search?query=data+table https://www.youtube.com/watch?v=fsWypGw5lGw