Abba, father, is a nickname for the sefirah of Hochmah, Wisdom.
The other nine sefiroth all are gathered into Hochmah on the ninth of Av. Abba, Hochmah, is the source of all Hesed, kindness, and Rahamim, Mercy.
The stage is set for HaShem's infinite Kindness and Mercy to shine out on the world.
We need to be ready to receive it.
May we merit to rebuild the beit HaMikdash speedily in our day(s).