Welcome to the forum, @Davi_Marcondes_Rocha!
original attribute is the original image, i.e. the raw pixel values of the image that are always included as a first slice in the feature stack.
See also the documentation:
When using grayscale images, the input image will be also included as a feature. In the case of color (RGB) images, the Hue, Saturation and Brightness will be as well part of the features.
An easy way to look at all the features computed is to save the feature stack from the Settings dialog of the Trainable Weka Segmentation plugin.