6,397 x 349 = 6000 x 300 = 1,800,000
I rounded 6,397 to 6,000
I rounded 349 to 300
Multiplying 6,000 and 300 is the same as multiplying 6 and 3 to get 18, then we tack on five zeros to the result. The five zeros come from the total number of zeroes in 6000 and 300
We have 3 zeros in 6000 and 2 zeros in 300, so 3+2 = 5 zeros total.
So this estimation technique allows you to multiply large numbers like this very quickly without needing a calculator or pencil/paper.
Since 6397 rounded down, and 349 rounded down too, this means the result of 1,800,000 is an underestimate of the true value which is
6,397 x 349 = 2,232,553
The estimate isn't too far off.