Machin

(une) machine = (surprisingly) a machine

E.g.:
une machine à calculer = a calculator
une machine à coudre = a sewing machine
une machine à laver (le linge) = a washing machine. Literally "a laundry-washing machine". Le linge = laundry, and it is optional in this expression.
une machine à laver la vaisselle = a dishwasher

In some cases, it is not even necessary to specify the type of machine:
taper (une lettre) à la machine = to type (a letter)
mettre le linge à la machine = to put the laundry in the washing machine

(un) machin, on the other hand, means thing(y)
C'est quoi ce machin ? = what's that thing ?

Machin = what's his name (feminine is Machine)
t'as vu Machin ? = did you see what's-his-name ?