It is when a bloke fucks a girls pussy or ass a short time after a previous bloke has fucked the same girl - with the first bloke shooting his cum inside her pussy or ass. As such, the second bloke is fucking something that is all wet and sloppy - hense 'sloppy seconds'.
It requires the girl not to clean herself after the first fuck, thus for the first blokes cum to still be inside her - acting as lubrication. In both cases, the blokes must fuck her bareback (without a condom) for a sloppy seconds to be experienced.
I myself enjoy a good sloppy seconds. There's nothing better than arranging for a girl to get fucked bareback, so I can be next - so as to shag her sloppy pussy or ass.
I prefer it when I get to watch the first bloke shagging, so as to see him climax - shooting his load inside her, then pulling out, with his cum dripping out! That's when I stick my dick inside her, pushing the first guys cum back into her - the feeling of her all wet, having just been shagged, is fantastic!