It's what's known as an alias in SQL.
The full SQL statement would be something like this:
FROM tblSales s
The bit in bold is saying; wherever you see 's' in the query replace it with tblSales.
As Ben says it reduces the amount of typing you have to do. Personnally, I never(*) use them as it reduces the readability of the query.
*Well, not quite never! Sometimes you have to use them to make the query work - for example if you need to bring the same table into a query more than once
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