We see at least two weed species in the photo, Hairy Bittercress on the bottom and what appears to be Ground Ivy (also called Creeping Charlie) on the top. The latter is a perennial weed, rooting at the nodes (joints on the stem) as it creeps. It can be difficult to remove when well-established, and might require the use of a systemic herbicide to kill the roots. More management information can be found on the linked page. Its presence might indicate that soil in that area is either too shaded or too damp (or compacted) to keep the grass healthy. If the area doesn't grow grass well despite efforts to improve the soil (such as if there is too much summer shade), then replanting the area after weed removal with an alternative groundcover might be more successful.

Miri