The first thing your prospect reads. It decides whether they read anything else.
The headline is a switch. Strong headline, the reader keeps reading. Weak headline, they're gone. You have maybe two seconds and a handful of words to make someone care enough to continue. The goal of a headline isn't to sell, it's to earn the next line of copy.
Legendary copywriter David Ogilvy said: 'When you have written your headline, you have spent eighty cents out of your dollar.' He wasn't exaggerating. Headlines account for most of the variance in marketing performance. Spend ten times as long on them as you think you should.