Identify weed in lawn
There are several weeds that commonly cover open ground or thin turf this time of year.
We can see mostly chickweed in your photo, but there is also hairy bittercress and possibly speedwell.
You can see and learn about them on this page from our website:
They are all what are referred to as 'winter annuals' due to their life cycle, as they will bloom, set seed and disappear by summer. You can pull or dig them out, though it's possible more seeds banked in the soil may sprout. (For what it's worth, they do have value to pollinators and wildlife (my chickens LOVE them), but it's good if you try and remove or treat them before they set seed- especially hairy bittercress, whose seed capsules dry and then shoot seed explosively throughout the area.
Thickening up your grass in those areas by overseeding can help shade them out in the future.
Here is our lawn page with more info on that: