They are two different method for assigning your starting stats for your character. On p13 of the Players handbook under section Determining Ability Scores. It’s not explicitly named “Standard Array” but you can assign these numbers to your stats: 15, 14, 13, 12, 10, 8. Point buy is explained in the section Variant: Customizing Ability Scores. You get 27 points to spend on your ability scores. Each score starts at 8 and the costs are: 8 - 0 points 9 - 1 point 10 - 2 points 11 - 3 points 12 - 4 points 13 - 5 points 14 - 7 points 15 - 9 points.