Can you please tell me what lawn weed this is
This looks like it could be globe sedge, one of the sedge lawn weeds. It is difficult to get rid of and requires persistence to remove. Mowing can keep it under control by removing the seed heads and there are herbicidal treatments available. Products containing sulfentrazone are effective against sedges including globe sedge. One such is Dismiss and another is Bonide Sedge Ender. There are others. Not all sedge herbicides will control globe sedge so read the labels carefully and apply as directed. Though many weed herbicides are formulated to not affect turf grass, over use of any herbicide can damage it. Also, be sure to observe the product warnings and follow the personal protection instructions for using it.