Double disk bundle: A smooth, closed manifold
$M \cong  DB^{-} \cup_L DB^{+}$
where
· $B^{±}, L$ smooth, closed manifolds
· $D^{l± +1} → DB^{±} → B^{±}$ smooth disk bundles such that
$S^{l±} → L \cong ∂DB^{−} \cong  ∂DB^{+} → B^{±}$ is sphere bundle.
In the above Double disk bundle, I do not understand what is base space and fibre of a double disk bundle are.
Please give some ideas to understand.

Thanks