Consider a good, old-fashioned spring-balance. The more
you hang from it, the larger the reading. Suppose it's
been calibrated, and when you hang one kilogram from it
the dial reads 1kg.
Now look at the arrangement we have here on the right. We
have two frictionless pulleys and a light, inextensible
string such as is only ever found in a physics textbook.
We put the (effectively massless) spring balance in the
middle, and we hang a kilogram from each end of the string.