it is a member of a family that has been split many times. Botanist can not make up their minds, Polygonum or Persicaria. Most are called some type of smartweed. all one word. If you don't like it is a weed if you like it is not. Most are annuals so this plant should die but each one of the little pearls contain a seed. If you allow to remain you will have more next year.

Even though to many these families are weeds you will find several types for sale at your local nursery in the spring.

I have about 15 different members of these families in my yard going from a 3 inch little one to one with 5 foot arching flower stalk that appear to have little onions hanging from the stalk. I find them very interesting.