PCR primer
If PCR is a photocopier, the primers are the two bookmarks that tell it exactly where the passage you want begins and ends. Without them, the copying enzyme would have no idea where to start, and you would copy nothing useful. The primers are how you point PCR at one specific stretch of DNA out of a whole genome.
A primer is a short single strand of DNA, usually about 18 to 25 bases long, designed in the lab to be complementary to one end of the target. A PCR uses a pair: a forward primer that matches the start of the target on one strand and a reverse primer that matches the other end on the opposite strand. During the annealing step they base-pair to their matching sequences (A-T, G-C). DNA polymerases cannot begin a new strand from nothing — they can only add to an existing 3' end — so the bound primer provides that starting handle, and the enzyme extends outward from it. Because both primers must find their exact matches, they define a target with high specificity: only the DNA lying between the two primer sites gets amplified.
Good primer design is the quiet heart of a working PCR. Primers that are too short or that accidentally match other places in the genome will amplify the wrong things; two primers that stick to each other form 'primer-dimers' that waste reagents. Their melting temperature sets the annealing temperature you use. Designing the right pair lets the same simple PCR machinery target a virus gene one day and a human disease mutation the next — you just change the bookmarks.
To amplify a 500-base-pair gene fragment, you order a forward primer matching its first 20 bases and a reverse primer matching the complement of its last 20 bases. Only the DNA spanning these two anchored sites — plus the primers themselves — appears as product, sharp and predictable in size.
A primer pair brackets the target and gives the enzyme a 3' start.
Primers do not just 'find' the target by magic — if a primer's sequence partly matches an unintended site in the genome, PCR will happily amplify that wrong region too, producing misleading bands.