You can think of this in two ways: either look at Maxwell's Equations, which are covered in fundamentals of physics II, or think about special relativity. I don't know Maxwell's equations and it's fairly advanced stuff so I usually think about relativity.
Special relativity tells us that as we accelerate something more and more it gains mass relative to a stationary observer, which makes it harder to accelerate more. Eventually this becomes so great that no matter how much energy we put in, the object's speed will increase by a tiny negligible amount only and never reach or exceed a certain speed. however, some particles have no mass (photons) so they can travel at this certain speed. This 'certain speed' is what we call the speed of light, c.
However, the speed of light isn't constant: it changes in different mediums such as the interstellar medium, the atmosphere or a block of glass. Check out this video: https://www.youtube.com/watch?v=CiHN0ZWE5bk