I studied about it and found interesting informations.
The Tau was cited on the Holy Bible in Ezequiel 9, 1-7 (some translations shows a cross and others show a Tau):
'The scourges of the city are approaching,
each carrying his weapon of destruction!'
Immediately six men advanced from the upper north gate,
each holding a deadly weapon.
Among them was a man dressed in linen,
with a scribe's ink-horn in his belt.
They came in and halted in front of the bronze altar.
The glory of the God of Israel
rose from above the winged creature
where it had been,
towards the threshold of the Temple.
He called to the man dressed in linen
with a scribe's ink-horn in his belt
and Yahweh said to him,
'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.'
I heard him say to the others,
'Follow him through the city and strike.
Not one glance of pity;
show no mercy; old men, young men, girls, children, women,
kill and exterminate them all.
But do not touch anyone with a cross on his forehead.
Begin at my sanctuary.'
So they began with the old men
who were in the Temple.
He said to them,
'Defile the Temple;
fill the courts with corpses;
then go out!'
They went out
and hacked their way through the city."
The Tau (or cross) represents the salvation of that people and the God's love for us.
God loves everybody, but many people don't match His love.
Jesus will come back and He will judge everybody with the perfect justice.