It's simply a list of all the measurements you make on an image . Let's say you measure your image and find out area = 12345, perimeter= 678, mean gray level=73, and average solidity of the blobs in the image = 0.7. Then you could make a feature vector
featureVector = [12345, 678, 73, 0.7];
Then you can do whatever you want with that, for example put it in an Excel workbook, or pass it on to some other function for further processing, for example to see what other feature vectors that you've cataloged in a database of human faces this particular feature vector matches best.
