This gist shows how to display the items in beamer one by one using \only overlay

displayonebyone.tex

\documentclass{beamer}
\usetheme{CambridgeUS}
\setbeamertemplate{items}[ball]
\usefonttheme[onlylarge]{structurebold}
\usecolortheme[RGB={205,173,0}]{structure} 
\setbeamerfont*{frametitle}{size=\normalsize,series=\bfseries}
\useoutertheme{infolines} 

%Pacakges
\usepackage{xcolor}
\usepackage{graphics}

\usepackage{amsmath}
\usepackage{xcolor}

\mode<presentation>{}

\begin{document}
    \begin{frame}{Motivation}
    \begin{itemize}[<+->]
        \item Travel time 
        \item Residence time
        \item Biology
    \end{itemize}

    \begin{block}{Definition}
        \only<1>{Travel time is the time required by the particle to move from one point to another point.}
        \only<2>{Residence time is the time required for the particle to get out of the system.}
        \only<3>{Biology is the study of living things.}
    \end{block} 

\end{frame}
\end{document}