Adding bleed is the way that you will be able to print on an entire sheet of paper (with no white borders).
Because the printing equipment cannot print to the edges of paper, it is usual to print the design on over sized paper and then cut it down to size (the standard oversizes are known as SRA sizes, SRA4 is a bit bigger than A4 SRA3 is a bit bigger than A3). However to cope with small inaccuracies in registration (the position of the image on the page) and with the limitations in precision of cutting equipment it is good practice to have the image larger than will be needed; hence the content 'bleeds' over the edge. Some of the image is then cut off and binned.
Figure 1 shows a design of a flyer. We often get designs sent to us like this, which we cannot then print as desired. To make sure that there were no thin white borders on this flyer we would have to trim it a couple of mm smaller than the original design. With the gap between the text and the edge of the image being so small at best it may look unbalanced at worst we may cut into the text.
Figure 2 shows the same flyer as we would print it with the cut lines shown as a black rectangle (don't put a black rectangle on your work :-). Notice how the writing is not only a good distance in from the edge of the complete image, but also a good distance from the edge of the intended cuts.....
Figure 3 shows the design as we would actually print it, without the black rectangle but with suitable crop marks that will serve as guides when guillotining. In this example there is a good deal of bleed but we ask for 5 mm at every edge. Remember that unless you really need your text at the very edge of your design then your text should be set in from the edge of the cutting line to reduce the chances of us chopping a bit off. Again 5mm should suffice.
It is not necessary for you to add the crop marks, we can do it, but if you do, then please note that each individual mark should be far enough away from the cutting edge to not touch each other. If your crop marks meet then you will probably end up with bits of them visible on your finished flyer. If you do not add the crop marks, it is sufficient just to let us know the intended dimensions of the final design, and the extra bleed that you have added....