Web certificates (TSL certificates) make a profitable business: they should be renewed from time to time. When the expiration date of a Web server has been reached, you receive such a warning.
When no money is involved, do not worry about it.
This site is run by Ivar at in Santiago de Compostela.
This site participates in the Amazon Affiliate program, designed to provide a means for Ivar to earn fees by linking to Amazon