The Bleed is the area to be trimmed and ensures that no unprinted edges occur in the final trimmed document.
The Safe Margin is the area between the graphics and text that are not meant to be trimmed.
The Trim lines help indicate where the product is cut down to the desired size and refers to the finished size of a product or end of the paper.
See these articles for more information...
Safe area: https://support.stressfreeprint.co.uk/hc/en-gb/articles/4403356784017