I wanted to point that, while redefining \theenumi is fine in most cases, the situation can be more complex if the counter is prefixed with a longer text, as by default the label of an enumerate environment is right-aligned at a fixed distance of the text leftmargin (\leftmargini here) and it can overflow into the leftmargin.
A better solution for this case is to use the versatile package enumitem. Demo:
\documentclass[12pt]{article}
\usepackage[utf8]{inputenc}
\usepackage[showframe]{geometry}
\usepackage{fourier}
\usepackage{enumitem}
\begin{document}
\renewcommand{\theenumi}{SSS\,\arabic{enumi}}
\begin{enumerate}
\item Particle Synthesis
\item Video analysis
\item Temperature Calibration
\item Video Demonstration
\end{enumerate}
\noindent With \texttt{enumitem}:
\begin{enumerate}[label=SSS\,\arabic*, wide=0pt, leftmargin=*]
\item Particle Synthesis
\item Video analysis
\item Temperature Calibration Temperature Calibration Temperature Calibration Temperature Calibration
\item Video Demonstration
\end{enumerate}
\end{document}

SSS 1., whereas you only defined:SSS\,\arabic{enumi}(without any trailing dot!). – Jan Jun 22 '21 at 07:13