Digital certificates are the digital equivalent (i.e. electronic format) of physical or paper certificates. Examples of physical certificates are driver's licenses, passports or membership cards. Certificates serve as identity of an individual for a certain purpose, e.g. a driver's license identifies someone who can legally drive in a particular country.
.Likewise, a digital certificate can be presented electronically to prove your identity or your right to access information or services on the Internet.
The same reason you trust what is stated in a driver's license: endorsement by the relevant authority (Department of Transport) in the form of a difficult to forge signature or stamp of approval. Digital certificates are endorsed in a similar manner by a trusted authority empowered by law to issue them, appropriately known as the Certifying Authority or CA. The CA is responsible for vetting all applications for digital certificates, and once satisfied, "stamps" its difficult to forge digital signature on all the digital certificates it issues, attesting to their validity (Source http://www.tcs-ca.tcs.co.in)