Some software might, for example, modify the firewall and break the security built in Tails. But other software like games or office tools are probably fine.
Software not [[officially included in Tails|doc/about/features]] might have not been tested for security.We also cannot provide support or documentation for it.
Right-click (on Mac, click with two fingers) on the name of the package in the search results and choose <span class="guimenuitem">Mark for Installation</span>.