It means your R1C1 reference setting has become enabled. If you haven't done it manually, then it means that you have opened a sheet which was R1C1 enabled and any sheet opened after that will be opened in R1C1 mode only.

To go back to A,B,C column notations which is called A1 style referencing.

1. Office Button > Excel Options > Formulas

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2. Uncheck the R1C1 reference style

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