You can think of DNA strands as two sides of a zipper. When unzipped, the two strands of DNA dissociate from each other. When zipped up, DNA strands reassociate. When heated, double-stranded DNA unzips, or dissociates, into single strands. When cooled, DNA reassociates to make double strands only when the strands find complementary bases with which to bond.
You can measure the amount of reassociation that occurs as a function of time by measuring the amount of double-stranded DNA. Diluting your sample will slow reassociation, as it takes longer for the complementary strands to find each other. Reassociation rates also depend on reassociation temperature. The higher the temperature, the longer it takes the DNA to reassociate.
If a DNA strand contains a repeating pattern, you can analyze this by measuring the rate at which heat-dissociated DNA reassociates. This rate is proportional to the number of times the genome contains that particular repeating pattern. The more repeats of the pattern within the genome, the quicker the strands will reassociate. The fewer the number of repeats, the slower it will take to reassociate. Given enough time, all of the DNA will eventually reassociate.