Are you talking about the floor function? Those aren’t brackets, those are… well, there aren’t keys for them on a standard keyboard. Basically, they look like brackets without the top horizontal marks. But it wouldn’t be “the greatest integer less than or equal to 0”, it’d be ”... less than or equal to x”.
So if
y = floor(x)
then for any x between 1.0000000 and 1.9999999<etc>, y = 1.
for any x between 2.0000000 and 2.999999999<etc>, y = 2.
and so on and so on.
To draw a graph of y=floor(x), you would draw a horizontal line between (0,0) and (1,0); between (1,1) and (2,1); between (2,2) and (3,2); etc. For each line, you make the rightmost portion of the line an open circle, to indicate that the line doesn’t actually include that terminal point.
If that doesn’t make a whole lot of sense, try this site that might explain it better than I did: http://www.mathsisfun.com/sets/function-floor-ceiling.html