The phrase comes from an interview of musician Warren Zevon by David Letterman. Asked what he understood now, having been diagnosed with terminal cancer, Zevon replied “Just how much you’re supposed to enjoy every sandwich.”
So enjoy the moment, and be in it. Carpe diem – seize the day – and do not fritter it away. And if your sandwich is a supermarket prawn mayo, enjoy every mouthful as much as if it were the finest smoked salmon.