OS X 10.11 El Capitan, released in 2015, will no longer receive security updates from Apple with immediate effect.
Usually Apple only supports the last three operating system versions and now 10.11 has dropped out of support. This circumstance affects all Macs built up to 2009 (with the exception of some iMacs from this year), which can officially be operated with 10.11 as highest OS X version. For Macs from 2010 on you should be able to upgrade at least to 10.13 High Sierra.
If you are using an older Mac with a maximum of 10.11 as your system version, you should now use at least a browser other than Safari, as it will not receive any updates either. Firefox and Opera offer an alternative. Furthermore you have the possibility to upgrade with a small patch to macOS High Sierra, which requires some technical skills.
