Traditional methods of generating ability scores for D&D, Pathfinder, and other d20 based systems are variations on rolling 3d6 in order to produce scores in the range of 3 to 18 that result in a playable character. What makes a set of stats playable is open to interpretation, but as a general rule the character's primary stat must be high enough to not hinder the character's class and advancement, and they shouldn't have more than one stat that results in an ability penalty. The problem with dice-based gneration, however, is that it is completely random: bad rolls are as frequent as good rolls, and the rolls for each stat are independant of one another. Even the popular method "4d6, drop the lowest" produces stats under 10 roughly 25% of the time, and while the "8+2d4" method avoids penalties it can still produce flat ability arrays. The biggest problem, however, is that simple die roll methods produce an enormous range of possible scores which can lead to huge disparities between stat arrays among PCs.
While point buy systems are designed to put all players on equal footing (and provide flexibility to customize builds), there are some players and GMs that prefer the element of randomness that comes from dice rolls, or see point buys as promoting undesirable min-maxing. This random ability score generator is designed to produce usable stat arrays and avoid wide variations between builds. It breaks the character stats into three groups—primary, secondary and tertiary—and each stat uses a different combination of dice to generate a score within a specific, limited range. It also prevents flat stat arrays that rank high in the point-buy system but are not usable in practice (e.g., a 14 and five 13's would be an 20 point build in Pathfinder, but it's probably not a build anyone would actually want to play).
Select the desired point buy range and then hit "generate" to obtain a build. As a general rule, only one primary stat is allowed to exceed 16, and only one stat will ever result in an ability penalty. Standard builds will never result in a score below 8, high builds will never produce a score less than 9, and epic builds will never produce a score under 10. Note that a secondary stat might exceed a primary stat, and a tertiary stat might exceed a secondary stat. This is fine, and to be expected. These groupings are arbitrary and merely ways of restricting the range of generated scores. The final stat array is what matters, and is printed in numerically descending order along with its point buy value.
If you prefer rolling dice yourself, simply set the desired point range and roll for each of the boxes, dropping the highest or lowest score for each stat as indicated. Use a d4 for a d2, and a d6 for a d3. You'll then need to manually calculate the point buy to ensure the scores fall within the target range.