The PR interval on an ECG measures the duration of electrical signal transmission from the SA node, across the atria and AV node, and into the ventricles. This interval spans roughly \( 0.12 - 0.20 \, \text{s} \). Its functions include:\begin{itemize} \item Assessing AV node conduction. \item Demonstrating the atrial to ventricular filling delay.\end{itemize}--- Final Answer:\[\boxed{\text{PR interval}}\]