Seconds is short for secondary image choice or secondary image selections. In culling through images the photographer selects images that represent the best for a given shoot. During this process some images are deleted (because they're unusable due to blur, misfires, camera setting errors, etc.) and the remaining are the seconds.
Let's suppose, for the sake of this example, that the photographer selects 10 of the 50 images to process. Of the 40 remaining, 10 of them are tests, errors or so blurry that they get deleted. That leaves 30 images as seconds from this photo shoot and they get segregated and saved. It doesn't stop here;
The photographer then turns over his primaries (the initial 10 he edited) to the client. Of those ten images the client will select only one for print. The remaining nine then become the client's secondaries. So now you know what secondaries are. Yeah, I know... long explanation.