Dna replication is said to be semiconservative. what does this mean?

Semi-conservative means:
The replication of  a DNA molecule, when the two complementary DNA strands are separated. It is usually fulfilled by  special proteins that unwind the molecule and expose the nucleotide bases. 
New complementary  DNA strands are then synthesized  by joining together deoxribonucleotide triphosphates, one at a time, and with the removal of a di-phosphate. At the end of the process, the cell as made two identical copies of all the biological information contained on the original parental, DNA molecule, but the two daughter molecules are Half-old and Half-new. Half the original DNA molecule is saved in the daughter molecules. This is why it's called Semi-conservative! Hope that this helped!

Aneutral atom must have the same number of neutrons and protons? no, they must have the same number of protons and electrons