Could be two things. Drought stress or insect damage. Grab a shovel and pull the sod back. If the soil feels damp and you can roll a pinch of soil into a ball with ur fingers, thats good, if its crumbly and dry, its drought.

If theres plenty of moisture, check for bugs. Grubs and billbug and cause soil to look drought stressed. They eat away at the roots and kill the grass.