Mindfulness is quite simply being present , to be "in the moment". This is usually achieved by a regular mindfulness practice which involves trying to be still, with a quiet mind. "Mindfulness" has become a big part of the wellness industry and with it there are a lot of incorrect connotations, you don't have to sit cross legged on the floor in harem pants to be mindful.
Absolutely anyone can practice being mindful, it does not matter if you are spiritual or a complete skeptic, there is no faith or belief system attached to the practice. All it takes is a little patience and some guidance.
Mindfulness is essentially meditation but without any faith, spiritual beliefs or motivations.