The focal length or range of a lens tells you how much of the scene will be captured in the shot, and how much magnification there will be. Measured in mm, a shorter focal length gives a wide angle effect that makes objects seems further away, while a longer focal length gives a telephoto effect that makes objects seem closer.
All lenses have an aperture range or "f-stop number", which tells you how much control you have over the focus. A lower f-stop number (e.g. f/2.8) lets more light into the camera, causing your subject to be in focus with the background blurred. They tend to have a faster exposure in low-light conditions, better for shooting indoors. A higher f-stop number, e.g. f/38, means the majority of the shot will be in focus, capturing all details.
Whatever lens you choose, it's important to pick one that's compatible with your cameras, as many brands create lenses that only fit their cameras. You also need to match by type of camera, as a DSLR lens will not mount to a mirrorless camera. In addition, you should check that the sensor is compatible, as a camera with a full-frame sensor will not mount to a lens made for cameras with a cropped APS-C size sensor.