At a guess, it would seem that they are happy with the quality of your work, so now they're interested in how you might be a good fit for their processes (especially as it's a senior position). Let's face it - as a senior, you'll be expected to work closely with the PM to deliver projects on time, react well together when unexpected problems occur, possibly work together to scope out projects together, decide on budget splits for project phases. Basically, a good 'fit' with the PM can be key to a good process, so it's understandable that they want you two to meet.

I'm clutching at straws here on your behalf, but in my experience (Head of design at a design agency in the UK) the PM will be a process driven person who likes things logged, planned and agreed ahead of time, so showing that you've also got a skillset in being organised and can plan well is going to impress. Becoming more senior isn't often about 'being a more talented designer', but being more aware of the project and it's deliverables - being able to make (sometimes tough) calls about how to approach a situation in terms of budget, timescales etc.

Hope it goes well. Good luck.