If xdot=Ax+Bu is a state space representation, what would a column of zeros in the A matrix depict in particular other than the system being singular in nature.
Apart from the fact that the matrix may be singular, the nullity of a colony generates non-cyclicity by one of the vectors of B, which implies the existence of non-controllable sub-space and hence the non-controllability of the system.
For more information see links and attached file in subject.
Operators, Functions, and Systems - An Easy Reading: Model Operators ...
https://books.google.dz/books?isbn=0821852655
Linear and Nonlinear Multivariable Feedback Control: A Classical ...
As said Adrian Gambier, only represent eigenvalues = 0. The systems with a integrator or integrator chain usually have that characteristic. Also you can check drift free systems, where A = 0.