Answer: the midline theorem of triangle: the midline of triangle is parallel to the third side of triangle and equal to half of the third side. The triangle midline theorem not only points out the positional and quantitative relationship between the midline and the third side of the triangle, but also provides a new idea for proving the positional and quantitative relationship (doubling relationship) between line segments, thus improving students' ability to analyze and solve problems. In addition, the conclusion of the theorem is the direct application of parallelogram judgment theorem and property theorem, which is widely used in graphic proof and calculation. Therefore, the understanding of this learning goal is that students should understand the concept of triangle midline, master the proof of triangle midline theorem, and experience mathematical thinking methods such as induction, analogy and transformation in the process of proof; Experience the process of exploring the triangle midline theorem, understand its internal relationship with parallelogram, understand the reasoning and argumentation methods of geometry, and realize the necessity and diversity of proof methods; Experience the process of reasoning, guessing and argumentation, and cultivate the ability of reasoning and argumentation.