The problem is, as stated in the comments, that you omit the necessary syntax for math mode. Because of this, TeX will at some point read a _ which is only a valid character in math mode. Hence, it will switch to math mode automatically, but since it does not know where the math mode should end, it keeps typesetting in math mode which results in this strange output.
It is very easy to solve this problem by using the correct syntax for math mode. Since you don't need pragraphs in columns one and three, you could also switch to another column defintion:
\documentclass{article}
\usepackage{tabularx}
\begin{document}
\begin{tabularx}{0.8\textwidth} { |
l |
>{\raggedright\arraybackslash}X |
c |
}
\hline
Place/Transition & Explanation & Time \
\hline
$T_1$ and $T_{2(n+1)}$ & Robot operation which relates to loadlocks. Transition $T_1$ indicates that wafer unloading from the loadlocks and $T_{2(n+1)}$ means that the robot loads the wafer to the loadlocks. & w \
\hline
item 31 & item 32 & item 33 \
\hline
\end{tabularx}
\end{document}

An alternative solution could be created with the help of the booktabs package, which can help you reduce the amount of borders between cells:
\documentclass{article}
\usepackage{tabularx}
\usepackage{booktabs}
\begin{document}
\renewcommand{\arraystretch}{1.25}
\begin{tabularx}{0.8\textwidth} {
l
>{\raggedright\arraybackslash}X
c
}
\toprule
Place/Transition & Explanation & Time \
\midrule
$T_1$ and $T_{2(n+1)}$ & Robot operation which relates to loadlocks. Transition $T_1$ indicates that wafer unloading from the loadlocks and $T_{2(n+1)}$ means that the robot loads the wafer to the loadlocks. & w \
item 31 & item 32 & item 33 \
\bottomrule
\end{tabularx}
\end{document}

Note that you might want to replace 0.8\textwidth by \linewidth if you want to extend the table to the width of a text column in a two-column layout:
\documentclass[journal]{IEEEtran}
\usepackage{lipsum}
\usepackage{tabularx}
\begin{document}
\lipsum[1]
\noindent%
\begin{tabularx}{\linewidth} { |
l |
>{\raggedright\arraybackslash}X |
c |
}
\hline
Place/Transition & Explanation & Time \
\hline
$T_1$ and $T_{2(n+1)}$ & Robot operation which relates to loadlocks. Transition $T_1$ indicates that wafer unloading from the loadlocks and $T_{2(n+1)}$ means that the robot loads the wafer to the loadlocks. & w \
\hline
item 31 & item 32 & item 33 \
\hline
\end{tabularx}
\newpage
Right column
\end{document}

T_1andT_(2(n+1))are supposed to be in math mode, right? While you're at it, shouldn'tT_(2(n+1))beT_{2(n+1)}? – Mico Mar 01 '23 at 06:56landc, respectively, as the column types for the first and third columns. – Mico Mar 01 '23 at 07:15landcstand for "left" and "center", respectively. – Mico Mar 01 '23 at 08:31