Cropping is the fastest way to change what a photo emphasises. A square crop suits profile pictures and product thumbnails, 16:9 fits video thumbnails and slide headers, 3:4 suits portrait posters and 9:16 suits vertical stories. Choosing the ratio first keeps the subject from being cut off later.
The frame starts as the largest rectangle of the chosen ratio that fits inside the picture. Zoom shrinks that frame, so the crop covers a smaller region at the same ratio. The position sliders move the frame across the spare space, from one edge to the other.
The output is a PNG at the cropped pixel size, so cropping never reduces quality. Convert or compress the result afterwards if the destination needs a smaller JPEG.