PSE stands for porn star experience. Like GFE it describes a register rather than a fixed list of activities, and it points at something more intense, more energetic and more performative than the girlfriend experience.
What it signals
Someone listing PSE is indicating that the booking is not aiming for the slow, affectionate tone that GFE implies. It tends to mean a more theatrical, higher energy encounter. Beyond that, as with GFE, the specifics are entirely down to the individual.
The same caution applies
No two people mean precisely the same thing by it. The term carries even more variation than GFE does, partly because it is used less often and partly because it borrows its name from a genre rather than a relationship. Reading it on a profile tells you the broad shape of what is on offer and nothing more.
Ask directly when you book, once, in plain words. Do not assume that because someone lists PSE, any particular thing is included.
Where it sits against GFE
Think of one scale. At one end, unhurried and affectionate. At the other, intense and performed. GFE and PSE are the labels people hang at either end, and a great many companions are comfortable somewhere in between, or at both ends depending on the evening. If a listing carries both, she will usually ask which you had in mind.