When you are creating a full page print ad for a publication you are given three sets of measurements. Here is what they mean.
This is where the magazine cuts. It essentially is the edge of the paper.
This is the area that will be a visual guide for you to know where all the important information of you ad needs to be. All type, logos, and anything that you don't want being chopped off should remain within the live area frame. If you have anything outside of it, you risk it being cut off if the cutting machines slip while chopping the magazines.
This is the "wiggle room" measurement. It will extend outside the trim area. Since no machine or human is perfect, we extend out all artwork to this measurement so when the magzine is cut, there won't be fine white lines on the edges. This applies really for anything that is printed and cut. If you are not given a bleed measurement, a safe bet is .125" all the way around.