Extra bases in front of the site, so the enzyme has something to hold on to. Enzymes cut poorly right at the end of a fragment, and it is a silent failure: the PCR works and the digest does not.
How many bases of your gene each primer carries. It is a starting point: the box below may move it a little on each side.
On, each primer may carry a few more or fewer bases than asked above so that the two annealing temperatures match, without adding warnings. Off, both carry exactly the bases asked for, whatever that does to their temperatures.
Rules out any pair whose junction would shift the reading frame or write a stop codon. Leave it off when the insert is not a coding sequence, or when the frame does not matter. Being in frame with a tag the vector already carries is always checked, box or no box.
On for the usual cloning strains (DH5α, TOP10), which methylate DNA. It warns when the bases around a site in your vector form a Dam or Dcm site, because some enzymes will not cut there. Turn it off for a methylation-free strain.