What does RBAR mean?

RBAR (pronounced REE-bar): Row By Agonizing Row

Databases are highly optimized to perform set-based operations.

For example, if you want to get the total amount owed on your open invoices, it will be much faster to do that in a query (a set-based operation) than by looping (row-by-agonizing-row).

GOOD: Set-Based Operations

SELECT Sum(AmtOwed) As TotalOwed
FROM Invoice
WHERE PaidOn Is Null

BAD: Row By Agonizing Row

Dim TotalOwed As Currency
With CurrentDb.OpenRecordset("Invoice")
    Do Until .EOF
        If IsNull(!PaidOn) Then TotalOwed = !AmtOwed + TotalOwed
        .MoveNext
    Loop
End With

GOOD: Set-Based Operations

BAD: Row By Agonizing Row

Further reading