The DAT is the "Dental Admissions Test." It is the dental equivalent of the MCAT (Medical College Admissions Test). It is an all day test that evaluates competence in academics and perceptual ability. These are attributes thought to indicate potential success in dental school. Here is a link from the ADA with lots of info on the test.
ADA information on the DAT
If you do well in chemistry, biology, and math, you will do well on the test. A long time ago, there was even a part where they would make you carve a large piece of chalk into something. This was to test manual dexterity. They have since done away with that part of the test.
Dentistry is not solely an academic endeavor. Yes, if you cannot manage chemistry, anatomy etc. you will not be admitted. If you ace the academic parts and do not have the ability to think in 3-D and rotate objects in your mind and express this, you will reach a point where you will not succeed at the more "practical" parts of dental education. In addition, if you are a total klutz with your hands and manipulating small objects, forget dentistry.
What matters in my experience on what gets you "in" to dental school is in this approximate order of importance:
2. Personality (people skills)
4. The DAT
Did I mention Grades? I have written extensively on this on other posts on the blog and in the comment section. If they see improvement in grades that is good. A "B" average from some schools is better than an "A" average from others. Still, a demonstrated record of achievement is something they look at seriously. They do not want to invest time, money, and effort on someone who will not complete the program. In addition, like any other highly desirable program, they want the best they can get. Dental school is competitive. So, a high DAT score will not get you admitted, but will help.
Here is some more information:
ADA publications on dental school