According to Lacan, the lesson of psychoanalysis is that the subject is inherently divided and can never be satisfied. We are plagued as subjects by the anxiety that our jouissance - our pleasure or enjoyment - is never enough. In other words, we are driven by an inherent dissatisfaction and sense of insufficiency.
We constantly have the sense that there is something more; we do not know what this is, but we have the sense that it is there, and we want it. This is the form of jouissance that Lacan identifies as phallic jouissance.
Phallic jouissance is that form of enjoyment that most of us experience most of the time; that is to say, just when we think we possess our object of desire - be that another person, a new possession or even a difficult idea we have been struggling to get hold of - we are still dissatisfied; we are disappointed and have a sense that our desire has not been fully satisfied. This sense of (dis)satisfaction that always leaves something wanting is precisely what Lacan calls phallic jouissance and defines the masculine structure.
A masculine structure is characterized by turning the Other into an objet a, and mistakenly thinking that the object can fully satisfy our desire. It is essential to keep in mind here, though, that phallic jouissance is not male in the sense that only men can experience it; it is experienced by both men and women and is defined as phallic insofar as it is characterized by failure.