belongs_to association sets up a one-to-one connection with another model, so each instance of the declaring model "belongs to" one instance of the other model.
For example, if your application includes users and posts, and each post can be assigned to exactly one user, you'd declare the post model this way:
class Post < ApplicationRecord belongs_to :user end
In your table structure you might then have
create_table "posts", force: :cascade do |t| t.integer "user_id", limit: 4 end