You first get the current value of projshowgrid, then you set the first bit of it (that's what the '&1' in the doc is saying) to 1 (on) by bitwise ORing it with 1 (binary ...000001) because

0 OR 1 == 1 and

1 OR 1 == 1

so it's set to 1 no matter of the previous state (the other bits stay the same because 1 OR 0 == 1 and 0 OR 0 == 0).

https://en.wikipedia.org/wiki/Bitwise_operation#OR
Sorry, Wiki probably does a better job explaining than me..