semiconservative replication
Imagine you have a precious two-page document and you need two identical copies. One clever way: pull the two pages apart, and for each old page write out a fresh matching page beside it. Now you have two documents, and each one keeps one of the original pages. That is almost exactly how a cell copies its DNA, and the name for it is semiconservative replication.
DNA is a double helix: two strands that pair up, A always opposite T and G always opposite C. To copy it, the cell unzips the two strands and uses each old strand as a template to build a new partner. Because each base only pairs with one specific partner, the old strand dictates the exact sequence of the new one. The result is two double helices, and each one is made of one old strand and one brand-new strand. The cell 'conserves' half of each original — hence semi (half) conservative.
This was not obvious in advance. In 1958 Meselson and Stahl ruled out two rival ideas: conservative replication (the whole old helix stays intact and an entirely new one is made) and dispersive replication (old and new bits are scattered through both strands). Semiconservative replication is the rule for every cell on Earth, and it is the reason a base sequence can be passed down faithfully — the old strand is the master copy that guarantees the new one is right.
Start with a helix written 5'-ATGC-3' paired to 3'-TACG-5'. Unzip them. The old ATGC strand templates a new TACG; the old TACG strand templates a new ATGC. Both daughter helices read ATGC/TACG — identical to the parent — and each carries one original strand.
Each daughter helix = one old strand + one new strand.
'Semiconservative' describes the whole helix's strand bookkeeping, not the bases — every individual base in the new strand is freshly made; only the old strand as a unit is conserved.