What does the hyphen mean for a block in Twig like in {% block body -%}?

The -%} means to trim whitespace.

See the Whitespace Control section of the docs.

http://twig.sensiolabs.org/doc/templates.html#whitespace-control

Reference:

http://stackoverflow.com/questions/17298902/what-does-the-hyphen-mean-for-a-block-in-twig-like-in-block-body