A more thorough explanation:

Definition: A future interest in a third person, who is intended to take after the natural termination of the preceding estate, that is not subject to any condition subsequent and is certain to acquire a present interest sometime in the future and will be entitled to retain the interest permanently.

Examples:

• "To A for life, and then to B" - B's future interest is a remainder indefeasibly vested.
• "To A for life, and then to B's children" - B's children have a remainder subject to open, but if the gift was "to A for life, and then to B's surviving children," then B's children would have a remainder indefeasibly vested.

These examples illustrate that a remainder indefeasibly vested is a future interest that is certain to become a present interest and is not subject to any condition subsequent. In the first example, B's interest in the property is certain to vest after A's life estate ends, and there are no conditions that could prevent B from taking possession of the property. In the second example, if the gift is to B's surviving children, then the interest is certain to vest in B's children, and there are no conditions that could prevent them from taking possession of the property.