OpenRepos is a community of software developers and distributors, that isn't officially supported by Jolla. Developers of this community make applications for SailfishOS (the operating system developed by Jolla), but since there is no official supervision by Jolla over these apps, some of these apps might not meet the quality requirements set by Jolla for SailfishOS. These apps might, for example use services within SailfishOS, thats' use is not permitted. These kind of applications _might_ cause problems with your Jolla. This is why we don't recommend their use.
You can install applications compatible with your Jolla from the Jolla Store.
If you don't know what OpenRepos (https://openrepos.net/) is and haven't installed an application called Warehouse, any issues relating to OpenRepos do not concern you. Open Repos -applications don't appear on your phone by accident, as you must see effort to enable and install them on your phone.
If your phone's Sailfish OS version is 188.8.131.52 or older, and you've installed the application called Warehouse (meaning that you use OpenRepos-applications), remove all OpenRepos-repositories before attempting to update your phone's software. Otherwise you risk damaging your device. You can read about the removal of these repositories from here: official-announcement-important-steps-to-do-before-updating