Neptun Tab, if added to your browser, is a custom dashboard featuring time, weather, and location widgets on your tab. However, it has been classified as a browser hijacker that will attempt to commandeer your browser search and homepage. Search results from this unreliable search engine will send you to malicious websites to get money from you or infect your computer further. It may also prevent you from changing your settings back.
Neptun Tab is usually installed through bundled software without you knowing. Remove it immediately.