During a process called transcription, DNA is temporarily split and different molecules that match the ones on one side of the DNA are put on an RNA base.
After transcription occurs, a new messenger RNA, or mRNA for short, is made.
After the mRNA is made, it leaves the nucleus, and does to a nearby ribosome in the cell's cytoplasm, and tRNA or transfer RNA, comes over to the ribosome.
Then, the ribosome connects the amino acids from the tRNA together in the order that the mRNA tells it to, and at the end of this process the amino acid chain is completed and a new protein is ready to be used.
However, sometimes the the DNA is changed or a mistake is made and the code the mRNA carries is changed.
If this happens, it can cause the protein made from this code to be made incorrectly, which can cause certain parts of the organisms body to be made incorrectly, and/or cause certain bodily functions to not be done correctly.