Meaning
The TAU has a shape of the greek letter (T) that is a cross.
St. Francis borrowed the Tau and what it meant to him from the Antonians. They were a religious community of men founded in 1095 whose sole function was to care for lepers.
The Tau is the habit of the Secular Franciscan living in the world.Francis was very familiar with these men because they staffed the leper house in Assisi and the hospital of St. Blase in Rome where Francis went to stay.
St. Francis was exposed to the Tau through the direct influence of the Antonians, but the greatest influence of all that made the Tau so dear to Francis, whereby it became his signature, was the Fourth Lateran Council.
Pope Innocent III opened the Council on November 11, 1215, with these words: “I have desired with great desire to eat this Pasover with you.” (Luke 22-15.) Innocent announced that for him, for the Church, and for every Catholic at the time, the symbol they were to take as the sign of their Passover was the Tau Cross.
He incorporated into his homily the statement from Ezekiel (9:4) that the elect, the chosen, those who are concerned will be marked with the sign of the Tau.
‘Go all through the city, all through Jerusalem, and mark a cross on the foreheads of all who grieve and lament over all the loathsome practices in it.’”
The TAU is also the last letter of the Hebrew Alphabet and it represents the cross as it was before the quote of Pilate was fixed. It is the signal the christians use when they say with the apostle:
“it is out of the question that I should boast at all, except of the cross of our Lord Jesus Christ, through whom the world has been crucified to me, and I to the world.” (…)
“Be champions of the Tau” 
This sentence became some of the most precious themes of Francis’ preaching. He must have taken them so deeply to heart that when Pope Innocent III ended his homily with “Be champions of the Tau”, Francis evidently took that as a personal statement and made the Tau his own symbol: a symbol for his order, his signature, painted it everywhere, and had great devotion to it for the rest of his life.
The TAU to St. Francis is a true expression of his life, as well as a reminder to us of our own Christian call and sincerity of response.
If you are marked with the TAU or you wear a TAU you are saying that you have commited to a continuous life of conversion according to Franciscan Spirituality. It does not mean you will suddenly be converted, but day-by-day, month after month, year after year, you keep your eyes in the Lord as your only goal and walk toward him only. (Written by Mary Margaret, 1989).
Shalom
The TAU is a signal of the chosen ones that God calls to a mission. Having St. Francis as an example, we are aware of that calling and we want to say yes to it.
We bring a seed of St. Francis inside our heart as a vocation and a community. Wherefore, the TAU is also a signal of election for us and the world. (Moysés Azevedo Filho).
In Shalom Vocation, each characteristic of the TAU has a special meaning:
It is a wooden TAU made by our own hands as a signal that we desire to life poverty in the Work and of the Work; to live only through what God does through our hands and work.
It is handed by a cord made of line so we remember what our Buwark said: “Everything in this life passes away. God alone is enough.”;
Once it was used by St. Francis, it shows the importance of this saint in our vocation;
It has the hebrew letters “SHALOM” written meaning the peace we look to be ministers and disciples of is the fully peace, in its hebrew meaning;
The word “SHALOM” written on a cross means that, for us, this peace is Jesus;
“SHALOM” is written in the Tau on fire simbolizing the unmutable choice of God for us and the vocation written on fire on our hearts.
Everytime we take it off or in we kiss it while we say: “Thank you Lord, for you have chosen me.”.
Taken from the book “Orando com a Bíblia e São Francisco de Assis”, by Jussara Lima Dias, from Shalom Community. Press: Shalom.
See more: https://comshalom.org/en/spirituality/