RSAGA provides access to geocomputation capabilities of SAGA GIS from within R environment. Having SAGA GIS installed is a (quite obvious) pre-requirement to use RSAGA.
In Linux x64 sometimes additional preparations are needed. In Linux SAGA as well as other software that would like to use SAGA modules usually searches for them in /usr/lib/saga, but if your Linux is x64, they usually will be located in /usr/lib64/saga. Of course you may set up proper environmental variables, but the most lazy and overall-effective way is just to add symbolic link to /usr/lib64/saga (or whatever a correct path is) from /usr/lib/saga:
:~> sudo ln -s /usr/lib64/saga /usr/lib/saga
Now no app should miss these modules.