Lord and Lady are honorifics used to address someone of authority. The exact authority needed is uncertain but seems roughly to be Warlord or higher, with use increasing with formality and the relative rank of the addressee. Being noble isn't a prerequisite for being addresses as lord.
People referred to as lord
Real World References
Lord is an honorific used to address a noble.
The difference between an honorific and a title
A honorific is used to preface a person's name as a sign of respect. A title is something a person posses that is not part of their name. People of different rank or title can share the same honorific. If Steward is a Baron he would be addressed as "Lord Steward". Something can be both a title and an honorific (i.e. "Prince") but it does not have to be the case.