What's the reason for the ashes?
Contrary to what some Fundamentalists may believe, the practice of marking one's forehead with ashes is not a pagan custom; rather, it has its roots in Old Testament practices in which ashes were symbolic of mourning, penance and humility.
We mark the beginning of Lent with ashes, indicating we mourn and repent of our sins. The ashes serve as a humble reminder that we are humans, not God. Creatures, not the Creator.
"In Bible times the custom <for mourning> was to fast, wear sackcloth, sit in dust and ashes, and put dust and ashes on one's head"
A Benjaminite fled from the battlefield and reached Shiloh that same day, with his clothes torn and his head covered with dirt.--1 Sam 4:12
Tamar put ashes on her head and tore the long tunic in which she was clothed.--2 Sam.13:19
Ashes also symbolize death and so remind us of our mortality. When the priest uses his thumb to sign one of the faithful with the ashes and says, "Remember, man, that thou art dust and unto dust thou shalt return," he is echoing God's address to Adam:
For you are dirt, and to dirt you shall return.--Gen. 3:19
This phrase also echoes the words at a Catholic burial, "Ashes to ashes; dust to dust," which is based on God's words to Adam in Genesis 3 and Abraham's confession, "I am nothing but dust and ashes" --Gen. 18:27.
Catholics are not required to have their foreheads signed with ashes. It is, though, strongly advised as a visible spiritual reminder that encourages us to adopt an attitude of prayer, repentance, and humility." source
Interestingly, there are now a number of Protestant denominations that are adopting this custom of marking the beginning of Lent with ashes. So don't assume that all ash-y folks today are Catholics!