Technically it's taught that floor is the greatest integer that is less than or equal to x and ceiling is the least integer that is greater than or equal to x.
So if x was 2.739492
Floor(x) would be the largest previous integer, which would be 2.
Ceiling(x) would be the smallest following integer, which would be 3.
If the number is already an integer then there is no change
i.e. if x was 5
Floor(x) would be 5.
Ceiling(x) would be 5.
Basically put it's rounding a number up or down without paying attention to the decimal part. Since Round(x,0) will round 5.51394 to 6 but will round 5.498389 to 5. If your application needs the number rounded up each time you would use Ciel(x), if it needed rounding down you rould use Floor(x)