Looks like Chenopodium album, aka Fat Hen or Lambs Quarters. Not a herb. An edible weed.

Lambs quarter is a weed and you don't want to let it go to seed, especially in your outdoor garden.

OK, when I say 'not a herb' perhaps I should explain that I am using the term in its common sense to mean culinary herb used as a flavouring, not herb in the sense of potherb ie vegetable nor in the sense of a green plant. It is not something the OP is likely to have received in a packet of 'herb' seed nor to have been sown intentionally.

All I have to offer is that, it's great for mixed greens in the spring, but I wouldn't probably intentionally plant it.

In other words - a weed!!