Excel references

1TeresaD_2020 , it's quite simple, absolute references (e.g. $A$1) will always refer to the cell specified when you copy and paste or fill down that formula to other cells.

Mixed references (e.g. $A1 or A$1) will always reference the relevant row (A$1) or column ($A1) when you copy and paste or fill down that formula to other cells.

Relative references will always move and shake when you copy and paste or fill across.  If you put the formula ... =UPPER(A1) ... in cell A2 and fill down for 10 cells, the references in all copied cells will move wit the fill down.

Circular references, you don't want those.  That's when 2 or more cells refer to each other in order to complete the calculation, it becomes infinitely recursive and cannot be resolved.  To test this, put this formula ... =A2+1 ... in cell A1 and this formula ... =A1+1 in cell A2.  They refer to each other and therefore, the calculation of the worksheet cannot be resolved.

Not sure if that helps but that's my take on it.