I suppose that you are playing with this, so

- 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).
- 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.
- 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).

responding to your comment questions:

Q1. the function `arcLength`

has a parameter `closed`

that let you choose the case (read the docs...)

Q2. Well, that I cannot prove, because I have not the image you have. Hint: display the thresholded image to see the area. You can also draw the contour on another image to see exact what contour is it using.

Q3. About this, I am afraid I cannot tell you much, but you can search on google, too. ;)

Please comment what I have not answered to and I will improve my answer, so it can be marked as answer ;)