The GARDENA gardening expert
From the photo you sent, I would say it is the Ajuga reptans "Atropurpurea" bugle. This mat-forming perennial has pretty blue flowers and is a member of the labiate (mint) family. It appears to have spread into your lawn. It seems to me, from looking at the grass blades, that you have a utility lawn, which is mainly made up of bunch-type rather than creeping grasses.
This means that there is more space between grass plants — enough for this bugle to move in and take hold. This perennial is not a big problem, however. A bit of scarifying easily removes it. Seedlings from offshoots may well take root again, but if you keep scarifying regularly, the unwelcome lawn invader will disappear over time. To finish off, it should be sufficient to remove any remnants with a rake.