This is pretty frightening stuff.
A woman—the Youtube uploader's mother, apparently—was filming the final moments of her race and wound up catching video and scary-loud audio of the first bomb explosion at Monday's Boston Marathon.
As soon as the bomb detonated, the woman turned around and ran in the opposite direction of the explosion. After having run approximately 26 miles and all the relief and exhaustion that entails, she had to snap back into action (though she likely got a nice shot of adrenaline).
The video cuts off before we hear or see the second blast, but according to the YouTube information she is safe, though she never finished.
Update 1:40 a.m.: This may actually be the second explosion. The Revere fire department has a picture posted on its facebook page that seems to show and explosion in the same area and notes that it is the second blast. You can also see smoke closer to the finish line. It's strange that people still seem relatively calm, but may also explain why the woman began filming when she did. The original headline has been changed to reflect this.
More coverage of the marathon bombing here.