DNA sequencing determines the order of the four bases—A, C, G and T—in a DNA molecule. That order is information, but a sequencing instrument does not automatically explain what every variant means for health.
Reading pieces of a genome
In a common approach called sequencing by synthesis, an enzyme builds a new strand from a template while the system detects which base is incorporated at each step. Large genomes are usually read in many pieces. Software aligns or assembles those reads, checks their quality, and compares them with a reference where appropriate.
Where uncertainty enters
Errors, hard-to-read repeated regions, and uneven coverage can leave gaps or uncertainty. A reported sequence is therefore a measurement with quality checks, not an infallible copy of a whole chromosome. PCR can make more copies of a chosen region before some analyses; sequencing asks the different question of what order the bases occupy.
Related: how PCR makes a selected DNA region detectable.