Definition of guardrails:
(n) :
(Canada, US) A strengthened barrier installed on the sides of a (usually major) road to keep vehicles on the highway and reduce the risk and/or impact of crashes.
(n) :
A rail set alongside a dangerous place as a barrier, to improve safety.
(n) :
(figurative, by extension) Any barrier designed to prevent accidents or mistakes.
(n) :
(rail transport) Synonym of check rail.