MATLAB is rich with documentation, you may read the respective functions documentation.
boundaries=bwboundaries(handImage1); % get;s the boudaries of the feature present in the image, the output is cell
x = boundaries{1}(:, 2); % select x coordinates of first boundary
y = boundaries{1}(:, 1); % select y coordinates of first boundary
hold on;
plot(x, y, 'black', 'LineWidth', 2); % plotting the first boundary
newImage = bwlabel(handImage1); % labelling the image
measurements = regionprops(newImage, 'Centroid', 'BoundingBox'); % get the properits of the region
% picking the centroid of the first label
xCentroid = measurements.Centroid(1);
yCentroid = measurements.Centroid(2);
subplot(2, 2, 4); % sub plotting
imshow(newImage); % shows image
title('Binary Image with Centroid Marked');
hold on;
plot(xCentroid, yCentroid, 'r*', 'MarkerSize', 10, 'LineWidth', 2); % plotting the coentorid
