Shamir's Secret Sharing Scheme (SSSS) is an implementation of a
threshold scheme for sharing a secret between third parties, and
requiring a threshold of those parties to collaborate to reveal the
secret.
Taken from the Wikipedia article about Secret Sharing:
In cryptography, a secret sharing scheme is a method for
distributing a secret amongst a group of participants, each of
which is allocated a share of the secret. The secret can only
be reconstructed when the shares are combined together;
individual shares are of no use on their own.
Shamir's scheme is provable secure: in a (t,n) scheme one can prove
that it makes no difference whether an attacker has t-1 valid shares
at his disposal or none at all; as long as he has less than t shares,
there is no better option than guessing to find out the secret.