combinatorics - Combinatorial interpretation for the identity $sumlimits_ibinom{m}{i}binom{n}{j-i}=binom{m+n}{j}$?

A known identity of binomial coefficients is that
$$
\sum_i\binom{m}{i}\binom{n}{j-i}=\binom{m+n}{j}.
$$
Is there a combinatorial proof/explanation of why it holds? Thanks.