Here is the complete statement:
They are closed two-dimensional figures. They are formed by three segments that connect three non-collinear points. They have exactly three sides, three angles, and three vertices. Each side is included between two angles and is opposite one angle. Each angle is included between two sides and is opposite one side.
For your comprehension, non-collinear points are points that do not form a straight line. Vertices are corner points where lines meet. Triangle is a 2-dimensional figure because you cannot generate a volume from it, only area.
