Thanks for your question. From these pictures it looks like it could be perennial rye, which is often included in the mix of species in your turf. If it looks like it's coming in aggressively, it may be that the rest of the lawn is weak. It looks like your mower blade is dull. Without seeing more of the lawn it's hard to evaluate what else may be going on. For sure, have the lawn aerated in the fall, and fertilized appropriately. Let us know if we can be of further assistance.