The warning is related to Russian malware (malicious software) called VPNFilter that can secretly install itself onto internet routers and collect data from an unsuspecting user. So far, it's estimated that 500,000 devices — mostly from Ukraine — have been affected.
Rebooting internet routers will “temporarily disrupt the malware and aid the potential identification of infected devices,” according to the FBI. The agency also recommends disabling your routers' remote-management settings and updating the password you use to access your router's settings.