So when Watson and Crick determined the structure of DNA, they came up with the theory that DNA was replicated semi-conservatively. Then two scientists called Meselson and Stahl did an experiment with two isotopes of nitrogen, N14 N15 (so N15 has one more neutron in it making it denser) in order to see if Watson and Crick was correct.
Here's a breakdown of the experiment
1) They had two samples of bacteria, one broth that had the lighter isotope (N14) and also the heavier isotope, N15. Whilst the bacteria replicated, it took up some of this nitrogen into its DNA.
2) Some of this broth was taken and spun into a centrifuge to show that the bacteria had incorporated the nitrogen into its DNA, as the broth with the heavier nitrogen settled at the bottom whereas the lighter broth settled at the top.
3) Then the bacteria in the heavy nitrogen broth was taken out and put into the light bacteria broth for ONE replication cycle and then this was spun yet again
4) They discovered that after one replication, the DNA settled in the middle. This is because HALF of the old double helix was mixed with HALF of the new nucleotides in the lighter broth causing it to set in the middle due to the two densities. This meant that it is SEMI CONSERVATIVE because (semi = half, so half of the DNA was made up of the old nucleotides, so HALF of the nucleotides are conserved)
Hopefully that makes sense.