Hello, is this a weed, or a plant/flower? It started coming out from our neighbor's garden, through the bottom of the fence!

Oh dear. I hate to be the bearer of bad news.
This looks like the dreaded , very invasive Japanese knotweed...Reynoutria japonica.

Grow through tarmac, and concrete.. Unstoppable...without chemicals.

https://www.google.com/search?...

Here's our database entry for Japanese Knotweed (Reynoutria japonica) where you will find member photos for comparison.

It is an extremely invasive species in many areas. More information and photos on these pages:
https://www.illinoiswildflower...
https://www.invasive.org/brows...

It is so sneaky of it to be so attractive!