Objects are required to establish and maintain their own internal coherence. The 'maintaining' part is done by ensuring self-consistency is restored after any operation completes (ex: by incrementing the link count after adding a new link to a linked list). The part about 'establishing coherence' is the job of a constructor.
Constructors are like 'init functions'; they build a valid object. The constructor turns a pile of incoherent arbitrary bits into a living object. Minimally it initializes any internally used fields that are needed, but it may also allocate resources (memory, files, semaphores, sockets, ...).
A constructor is like a 'factory': it builds objects from dust.
