The law of segregation was a genetic law stated by Gregor Mendel. Genetic information for traits comes in pairs of chromosomes. Lets say one chromosome has a trait B while the other one has a similar but different trait b. During meiosis the pairs are separted (segregated) into two new cells one with B and the other with b. When these cells are reunited you can get BB, Bb, bb offspring. Mendel could not see meiosis happening, he just guessed it worked that way because it fit his observations of what happened when he crossed various kinds of pea plants.