Before you can use Yaffs, you must accept its license. Yaffs is not covered by the standard eCos license. You will be reminded of your license to use Yaffs when you install it.
Most users will only have access to Yaffs under the GNU GPL. This costs nothing to license. However this usually means that if you ever distribute your application, you must do so under the GPL. This requires you to publish or otherwise make all of your application, eCos and everything else you link with it available as source code. For full details refer to the text of the GPL (v2).