Mendel's Law of Independent Assortment says that the alleles of each gene sort independently of each other during meiosis. For instance, let's imagine a person is heterozygous (has two different alleles) for eye color (Bb) and also for hair color (Rr). According to the Law of Independent Assortment, this person should produce gametes containing BR, Br, bR, and br in equal proportions.
In reality, let's say that these two genes are side-by-side on the same chromosome, and our test person has one chromosome that is BR and another that is br. Because the chromosomes, rather than the individual genes, are being distributed to the gametes, most of the time BR will stay together, as will br. The item that is really subject to independent assortment is the chromosomes.
Crossing over, where chromosomes exchange sections, does keep mixing the genes, but it is a slower process of shuffling than true independent assortment would be.