What defines a deviation vector

Main Question or Discussion Point

In the last paragraph in Wald p. 224, he states that if one considers a timelike geodesic $\gamma$ with tangent field $\xi$ and $p \in \gamma$ and now looks at the congruence of geodesics passing through $p$, then every Jacobi field which vanishes at $p$ is a deviation field for this congruence.

Questions:

1. What properties actually defines a deviation field?
2. Why is the statement above true?