It's a cleaver - known as Sticky Willy to his friends
Aka goosegrass. Can be cooked and eaten like spinach. Never tried it myself, but I don't like spinach. It's an annual, so pull it all out quickly before it sets seed.
food Source for some caterpillars including the beautiful Hummingbird Hawk Moth. I pull it up in my main flower borders but let it grow in our ‘wild’ corners.
Be sure to get as much if not all out, it WILL regrow from even the tiniest piece. I have had it and it took quite a will to get rid of it.
Even if you can't get to the roots, it's worth just pulling out using a rake or hoe etc. to hook it out from few feet away. The main thing is to get rid of the bulk of it before it sets seed, or it will get worse and worse every year as the seeds (which are viable for up to 5 years) accumulate in the soil.
East facing, top of a hill clay-loam, cultivated for centuries (7 years by me). Birmingham