the leading strand
At a moving replication fork, one of the two new strands has it easy: it can be built in one smooth, continuous run, chasing the fork as it opens. That lucky strand is the leading strand. Picture a road crew laying pavement right behind a machine that clears the road — they just follow it forward without stopping.
Why can one strand be continuous? Because DNA polymerase can only build in the 5'-to-3' direction, and the fork is opening in a single direction. On the strand whose template runs 3'-to-5' into the fork, the new strand's required 5'-to-3' growth points the same way the fork is moving. So as helicase peels the strands apart, the leading-strand polymerase simply follows right behind, adding nucleotides continuously toward the fork, needing only a single primer to get started.
The leading strand is one half of the answer to a key puzzle: the two parental strands are antiparallel (running in opposite directions), yet both must be copied by polymerases that all go 5'-to-3'. One strand's geometry happens to line up with the fork's motion (leading), and the other's does not (lagging). Understanding the leading strand only makes sense alongside the lagging strand — they are the two faces of the same directionality constraint.
At one fork, the leading strand is laid down toward the fork in a single piece after just one primer. At the same fork, the other new strand (the lagging strand) must be made the hard way, in many short backstitched pieces — same fork, two very different jobs.
One continuous strand, primed once, following the fork.
'Leading' and 'lagging' are defined relative to one fork; because a bubble has two forks moving opposite ways, a strand that is leading at one fork is lagging at the other — the labels are local, not fixed to a whole strand.