What is an anvil and what does von bulow mean when he says germany will be a hammer or anvil?

Anvil is a tool used by blacksmiths, and it is the surface on which the object is placed which is worked on with a hammer - stroked with a hammer.

von Bülow meant (in 1899) that Germany will either have to colonize others (be the hammer) or it would have to risk being colonized itself (being the anvil), hence showing the need for Germany to participate in colonization.