It would mean instead of any sharp business attire (suits, blouses, shirts, ties etc) you could go for a nice casual pair of chino's and a short sleeved shirt - if you are a guy or for ladies less formal dress, skirt, blouse, top etc.
I don't personally think business casual means Jeans, but some firms do, so I would check on that one personally.
Something that is still appropriate to work in the office in, but you wouldn't wear it to a meeting is how I always judge it.
HTH?