what does "neat causal wear" means?

I would not wear jeans, try a button down shirt or a blouse and slacks or a modest skirt. You can throw on a cardigan or pullover as well. Non-blue-jean pants will also work, as in corduroy or chinos for example.
Shoes should be nice leather boots (desert, chelsea, boat shoe etc) leather shoes, or flats.

Once you work there a a few days you will get a feel for what is acceptable.

Slacks, shirt with a collar, no tennis or running shoes.
Just my humble opinion.

I would start with something like these (without the fitted suit jackets, because casual):
http://twentyproject.com/wp-content/uploads/2013/02/business-casual.jpg
http://www.trendvogue.net/wp-content/uploads/2013/01/Fashion-Item-for-Men-Business-Professionals.jpg (Sorry, I couldn't find one where it showed men it multiple looks. And no need for the tie.)

The basic look is slacks and button up, and it'll give you a nice safety net where you might feel a little overdressed for a day or two but you'll be well within dress code while you scope out how others are dressed. Tip: take your cues from people you respect, like a higher-up whose position you would like to have one day.

just wear whatever you want as long as its neat

Casual trousers or a skirt and long sleeved T-shirt.

Jeans (however tidy) are seen as scruffy.

Neat, not frayed, clean, and properly fitting.

Jeans or casual black/navy pants would be suitable. If possible, contact your employer before you start to get clarification. The more you know, the more you can do.

yeah jeans, t-shrirts, sneakers.leggings.. nothing too fancy