Lightspeed is the speed of light, and it is very, very fast. In fact, it is the fastest speed that anything can travel. It is so fast that it is difficult to even imagine how fast it is.
To give you an idea, light can travel around the entire Earth in just a little over one second. It can also travel from the Sun to the Earth in just a little over eight minutes.
Because light is so fast, it is used as a way to measure distance in space. For example, if we want to know how far away a distant star is, we can measure how long it takes for its light to reach us. The longer it takes, the farther away the star is.