It's floor(_) - as in, floor(1.999) = 0, but floor(2.001) = 2. If you look carefully the upper flange of the [] square brackets is missing, which makes it a floor.

https://en.wikipedia.org/wiki/Floor_and_ceiling_functions

It's the floor function https://en.wikipedia.org/wiki/Floor_and_ceiling_functions

(Had to check the TeX code to figure that out, MathJax let you do that with a right click on the equation.)

Thank you for asking this question. I didn't understand the notation either

Sorry about this, I added an explanatory note to the article.