Special relativity is a theory proposed by Albert Einstein in 1905 that describes how time and space are interconnected. It states that the laws of physics are the same for all observers, regardless of their relative motion. One of its key principles is that the speed of light in a vacuum is constant and does not change, no matter how fast an observer is moving. This theory leads to some surprising effects, such as time dilation, where time appears to pass more slowly for objects moving at high speeds compared to stationary observers. It also introduces the concept of length contraction, where objects appear shorter in the direction of motion when traveling close to the speed of light.