"roofee" wrote:
what this is this mean in excel (1.66E06)

I assume you mean 1.66E+06.

It is called scientific notation. It is read "1.66 times 10 to the 6th",
meaning 10 multiplied by itself 6 times (1000000). The actual numeric value
can be anywhere between 1655000 and about 1664999.99999999.

Implicit in your question might be "why does the number appear this way?"
and "how do I avoid it?".

Presumably, the cell is formatted as Scientific with 2 decimal places.
Click on Format Cells Number. If you want a different format, select
General or perhaps Number with some number of decimal places.

As for how it got that way without your knowing it, perhaps you imported
data that included the number written as 1.66E+06.

PS: If the cell truly displays 1.66E06 without the "+", I would guess that
the cell was formatted as Text when data of that form was entered. In that
case, you might want to change the format to General or a numeric format,
then press F2 and Enter.