What does * (star) integrity axiom mean in the Biba model?

The Biba model defines a set of security rules, the first two of which are similar to the Bell-LaPadula model. These first two rules are the reverse of the Bell-LaPadula rules:

The Simple Integrity Axiom states that a subject at a given level of integrity must not read an object at a lower integrity level (no read down).
The * (star) Integrity Axiom states that a subject at a given level of integrity must not write to any object at a higher level of integrity (no write up).
Invocation Property states that a process from below can not request higher access; only with subjects at an equal or lower level.