Although the photos are a bit out-of-focus, this appears to be a type of Chickweed (there are several species in our area, each looking a bit different). They are common lawn weeds in areas where turf is less vigorous and dense, and they sprout in late summer or early fall. As such, pre-emergent herbicides are most effective when applied at that time; it is too late now. However, you can spot-treat areas of weed outbreaks with a post-emergent, and the details on this page will specify which active ingredients to look for (you can also check individual product labels to verify Chickweed(s) are listed as controlled): https://extension.umd.edu/hgic/topics/common-chickweed

The herbicide label will give instructions as to how to apply and if any re-applications are recommended. If you prefer not to use chemicals, you can hand-pull them; some Chickweed species are annuals and thus have a minimal root system that will be easy to remove. Performing a soil test to guide lawn care going forward and keeping the lawn dense via overseeding (best done in fall, starting next year as it's too late now) will both help keep weeds at bay in the future. Information on soil testing and our page on lawn overseeding can be found on these pages:
https://extension.umd.edu/hgic/topics/soil-testing-and-soil-testing-labs
https://extension.umd.edu/hgic/topics/lawn-renovation-and-overseeding

Miri