This is an 18x24 poster. Since the design goes to the edge of the paper, this poster requires a bleed. The bleed margin is currently included, making it's present size 18.25"x24.25".

As you can see, this adds an extra 1/8 of an inch on all four sides. If this poster had white edges, it would not require a bleed margin.
This is an example of a bleed margin. The red lines indicate where the poster will be trimmed. Everything outside of the red trim lines is the "bleed" and will not be present on the final product.
Here is the poster after the bleed has been trimmed off. This is the final size - what the poster looks like when it is shipped to you.
The green marks indicate the "safety area". Anything crucial to your design should be left off this area.
Here is the finished product. The final size is 18x24.