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I suppose that you are playing with this, so

1. contourArea is returning the area inside a contour (ofcourse, it should be closed, otherwise, it will consider it closed, so if it is a line it will consider the area inside the contour defined by the line if the first point and the last point are linked).
2. As mentioned in the docs, the unit is pixel. If you want to get more deeper in the theory, then see Green's theorem; but for a better understanding, if the contour is a triangle, then ofcourse its area is very probably not an integer number.
3. arcLength is measuring the length of a contour in pixels, ofcourse. Think that the contour is composed of points, so between each 2 points, there is a line, so that segment's length can be computed if the coordinates of the 2 points are known; and the coordinates are in pixels (with (0, 0) in top left corner of the image).

I suppose that you are playing with this, so

1. contourArea is returning the area inside a contour (ofcourse, it should be closed, otherwise, it will consider it closed, so if it is a line it will consider the area inside the contour defined by the line if the first point and the last point are linked).
2. As mentioned in the docs, the unit is pixel. If you want to get more deeper in the theory, then see Green's theorem; but for a better understanding, if the contour is a triangle, then ofcourse its area is very probably not an integer number.
3. arcLength is measuring the length of a contour in pixels, ofcourse. Think that the contour is composed of points, so between each 2 points, there is a line, so that segment's length can be computed if the coordinates of the 2 points are known; and the coordinates are in pixels (with (0, 0) in top left corner of the image).