A lot of new video content has unwanted footage at the start and end; usually recorded when setting up and stopping your recording device. 'Topping and tailing' involves trimming the start and end of that video.
Most video editors have the facility to top and tail videos. Some software will have cutting tools, others such as ReCap have sliders which you can click and drag to remove unwanted footage. When recording a new video, try to leave a couple of seconds of silence at the start and end to make topping and tailing easier.
