Manipulate is any modification made to text, images, sounds, video, or other data. Often these modifications are made to deceive the viewer that it is something that it really is not. For example, someone may manipulate an image of a woman and change the color of her eyes, make her thinner, etc.
To manipulate something on the computer requires an editor. For example, a text editor would allow you to manipulate text, an image editor manipulates images, a sound editor manipulates sound and a video editor manipulates video.