LED stands for Light Emitting Diode. This is a bit of semiconductor (like a silicon chip used in computers) which produces light when electricity passes through it. It was invented in the middle of the 20th Century and was first used in low power applications such as pocket calculators. Recently the technology has evolved to be able to produce replacement bulbs for household electrical circuits. This technology uses around 10% of the electrical power of a traditional incandescent light bulb to generate the same lamp brightness. If a bulb can use a tenth of the power then it really is worth replacing all those old ones.
There are two main bonuses of LED lighting. One is that their lifetime is much longer than other technologies. The other is that, because they use less electrical energy, they produce less heat. Lower levels of heat from lighting means that in hot climates there is less cooling required from air-conditioning equipment.