Meselson and Stahl concluded that DNA replication is semiconservative.
Semi-conservative type of replication refers to replication of DNA molecule in which newly formed DNA molecule contains one new and one old strand. During the replication, two strands of DNA unwind from each other (enzyme helicase), and each of them acts as a template for synthesis of a new, complementary strand. So, as a result, two DNA molecules that are formed are with one original strand and one new strand.
Mechanism shown by this experiment is today universal mechanism of replication, shared by all organisms on the world.