# What does it mean if a number has a remainder of 1?

I have this problem:

linesize = 20

counter % linesize == 1 // what does this line mean?
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It means the the variable counter is of the form $x.20 +1 (x \in \mathbb{N})$ or in other words, counter is One more than a multiple of 20 (21,41,61 etc.)