How to use \leadsto in LaTeX2e?

The package amssymb provides \leadsto and many other symbols.

A good resource for finding symbols and the packages providing them is the "Comprehensive LaTeX Symbol List" that probably is already on your disk as part of the TeX distribution.

See also How to look up a math symbol

\leadsto is a math sign and for this only available in math mode.

You must enclose it in dollar signs $...$ (or \(...\) for inline math or use it in display math like \[...\] or the {equation} environment

[…] gestures recognition. $\leadsto$ Multimodal input system & […]

If you find that you often use this in text mode, I would define a macro as below using the xspace package and always use Leadsto:

\documentclass{article}
\usepackage{amssymb}
\usepackage{xspace}

\newcommand*{\Leadsto}{\ensuremath{\leadsto}\xspace}%

\begin{document}
    Now you can use \Leadsto outside in text mode or $\Leadsto$ in math mode.
\end{document}

The xspace command decides whether to insert a space or not -- see the package documentation. If you don't want to load another package you can use it as \Leadsto{} for the case where you want to insert a trailing space. Alternatively, you could hard code in a space at the end of the definition as I can't think of a case where you'd not have have text following this character -- altough, I prefer the xspace solution.