Mathematically, it means that the number of lines read is bounded by 10*M, where M is some constant. So it's basically equivalent to saying that it's O(1).

I'm guessing that intention was to say "around 10 lines", though it kind of stretches the definition if we're being picky.

I normally see that from engineers using "O(x)" as "approximately x" whenever it's clear from context that you're not actually talking about asymptomatic complexity.

I think the O means order of magnitude. It looks like Big O notation, but O(10) would collapse to O(1) and OP is not talking about efficiency anyway.

"about 10"