The resolution of a video corresponds to the visual quality of each image, calculated in number of pixels high and number of pixels wide.
What are the practical consequences?
With a better resolution, the user can zoom in on the image and still get a sharp rendering.
The higher the resolution, the heavier the video will be (i.e. the more storage space it will take on your phone or computer and the longer it will take to upload/download on the internet).
To be able to film in HD, Full HD or even Ultra HD (see below), you need a camera of increasing quality. On older phones, the cameras are not qualitative enough to film in Full HD or more.
What are the standard resolutions on the market?
An "HD" resolution = 1280 pixels wide x 720 pixels high (1280x720 or 720p)
A "Full HD" (High Definition) resolution = 1920x1080 or 1080p. This is the default resolution for iPhones 7 for example.
An "Ultra HD" resolution = 4k = 3840 × 2160 or 2160p.