Mr. Weaver is long dead, but he got into the scope/rings/bases business before WW II. Through various iterations, the products and name lives on.
The dealie on top of a Bushmaster "flat top" is an example of a Picatinny rail, if you check out their website. SFAIK, all these rails accept Weaver or Weaver-style rings. I believe the name comes from the arsenal of that name where the idea was dreamed up; not really sure.
These rails also can be mounted alongside or below the forearm/barrel and all manner of gizmos can be attached.
FWIW, Art