Excel Cell Reference; '=+' meaning

Hello friends here,

I wish to ask a question about Excel cell referencing cell value from another work sheet.

1. The value in the cell was like this:
=+'Sheet2'!B3

2. In fact, Sheet2 stores a Query Table that has its values returned from the SQL Server Database.

3. And my problem is: I change the value on the cell to：
='Sheet3'!B3
or even ='Sheet2'!B3, without that "+" character,
and the value fail with literally "='Sheet2'!B3", not the value it is expected to get reference.


Could you possibly tell me what's the meaning of "=+" and how to use it correctly?
Thank you very much!

Nick.