Understanding Bash if statement that invokes a command

Does anyone know what this is doing:

if ! /fgallery/fgallery -v -j3 /images /usr/share/nginx/html/ "${GALLERY_TITLE:-Gallery}"; then
  mkdir -p /usr/share/nginx/html

I understand the first part is saying if /fgallery/fgallery directory doesn't exist but after this it it not clear for me.