A custom post_type is a type of post that has a unique name, like ‘event’ or ‘book’ depending on the type of object you are needing. This can be anything. The post_type ‘post’ is a built in WP post_type and concerns only blog posts in the “Posts” panel of the admin area.
Add 'post_type' => 'your_post_type_name_here'
for the post_type you are needing to your WP_Query() arguments.
Hope this helps!