Before moving or resizing anything in Paint, you select it first — the same idea as selecting text before formatting it.
Rectangular selection grabs a box-shaped area. Free-form selection follows a hand-drawn outline instead, for grabbing an irregular shape without the surrounding rectangle.
Once something is selected, drag inside the selection to move it, drag a corner handle to resize it, or use the Rotate button in the Ribbon to turn it.
Crop is selection, not a separate tool
Cropping is just a selection plus one click: select the area you want to keep, then click Crop — everything outside the selection is discarded.
Image → Rotate and Image → Flip work on the entire canvas, not just a selection — useful for fixing a photo taken sideways, or mirroring an image horizontally.