"Nuclear poisons" refers to neutron-absorbing substances produced through transmutation. When the reactor shuts down, it will continue to produce these for some time after the shutdown -- Xenon-135, particularly.

This proves problematic during reactor startup. Since it's absorbing neutrons, the controllers would need to retract the control rods further out than during steady state in order to achieve a replication rate of 1.0. However, xenon-135 stops being a nuclear poison after absorbing its neutrons... which means the replication rate will increase, and the reactor might run away.

This is part of what happened in Chernobyl, though they'd also disabled most of their safeties. (And dismantled the others.)

Regardless, nuclear poisons are something to keep a very careful eye on.