What is this and how do I get rid of it?
This looks like mature yellow nutsedge. https://extension.umd.edu/hgic/yellow-nutsedge
This is not an easy weed to control. If this is spotty throughout your lawn, you can use a selective herbicide mentioned in above link. You may need several applications. Follow label directions. If you need to overseed, see the label for timing.
If this is a solid area of nutsedge, you can spray with a non selective herbicide such as glyphosate. This kills everything. You may need several applications. Follow label directions for seeding. You can seed up until mid October.
See our website for lawn renovation and seeding http://extension.umd.edu/hgic/lawns/lawn-renovation
Monitor next season and control as needed when the nutsedge is immature.
Thank you for your quick response!