Answer: It sounds like poa annua or annual bluegrass. Poa annua is difficult to eradicate once established. If you want to dig it out and reseed you can do this (or use Round-Up to kill the grass). Creating a dense lush lawn is the best way to keep poa annua out of the lawn in future.
general lawn care tips: fertilize in late spring, fall and late fall; water infrequently but deeply when you do water; mow the lawn high (2.5-3.5" tall after cutting) and mow frequently. try not to cut more than 1/3 of the length of the grass off during any one mowing. Mulch mow and return clippings to the lawn.
Best wishes with your lawn.
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