Thanks for the question. There could be a number of causes. You did not say when the brown spots appeared. Yellowing or browning grass in August is an early symptom of white grub damage. Here is a link to more information and treatment for Japanese beetle grubs https://extension.umn.edu/yard-and-garden-insects/japanese-beetles#when-to-treat-1592860

European chafers are another possibility, less common but more damaging. Here is a link for more information: https://extension.umn.edu/lawn-insects/european-chafer

Another possibility is heat and drought stress. Here is a link for more information: https://apps.extension.umn.edu/garden/diagnose/plant/turf/irregular/other.html

Another possibility is crabgrass, which is an annual, dying and turning brown in the fall by the time of a killing frost. This would most likely be in areas along the edges or in high traffic areas not typically in isolated patches. Link to more information: https://extension.umn.edu/weeds/crabgrass

Here is a link to more information on growing a thick, healthy lawn, which is important to discourage weeds: https://extension.umn.edu/planting-and-growing-guides/lawn-care-calendar

I hope this helps you have a beautiful lawn.

Paul -Master Gardener volunteer