[技术][流体][询问] 这句话说的是流体的什么性质呢

[Sam]

看流体的书时，碰到下面几句话：
A fluid is determined using two attributes:
1. it is a continuum.
2. only pressures forces can act at the surface of a fluid in rest,
therefore neither normal nor shear forces can occur.
请教一下大家，第二句话怎么理解？