The oblique angle determines the forward or backward slant of the text. The angle represents the offset from 90 degrees.
Here below an example of normal text, one of text slanted by 30 degrees and a further one on a 30 degrees angle.
Entering a value between -60 and 60 makes the text oblique. A positive oblique angle slants the text to the right. A negative oblique angle slants the text to the left.