When you are creating a flyer, the bleed area is the area on the outside of your flyer that is trimmed after printing. Setting up the bleed area in Photoshop is important because it ensures that none of your design is accidentally cropped once it is printed.
To add a bleed in Photoshop, you will first need to make sure the rulers are visible by going to View > Rulers and enabling snapping by going to View > Snap. Then click on the ruler and drag a guide to the edge of the document on all four sides.
From here, you can add the bleed by going to Image > Canvas Size and changing the dimensions to your desired bleed amount. For example, if you were to work within an A5 document with the dimensions of 154 x216mm, you would also need to set a 3mm bleed and a resolution of 300 pixels per inch. You can then use the Move tool (V) to create the horizontal guide by moving your cursor across the top of the ruler. Ensure the guide is set 3mm at the top and bottom of the horizontal and vertical axis.