In article 2 of question 63 (part 3 of the Summa), Thomas investigates what the baptismal character actually is. He relates it to our ability to participate in divine worship. "Divine worship consists either in receiving some divine things or in handing them on to others. And a certain power is needed for both of these activities...this is why character connotes a certain spiritual power ordered to those things which pertain to divine worship."
He goes on to specify that the power is a certain type of instrumental power. By that he is referring to the idea of instrumental causality. For example, if I write with a pen, the pen is the instrument I use, so it is an instrumental cause of my writing. This principle of instrumental causality is an important one in Thomas' theology of the sacraments. He sees the divinity of Christ working through the humanity of Christ as the cause of the power of the sacraments. The sacraments themselves are an instrumental cause, but one that is separate, not conjoined, to Christ.
So the character is a kind of spiritual power. Baptism is the doorway to the other sacraments because it enables us to receive them, through the baptismal character.
We could also note that the character in itself, like any power, can be used well or badly. It is used well by those of the baptized who take seriously their Christian commitment. It is used badly by those who, instead, lead a sinful life, a life apart from God. But once we have the baptismal character, we can never lose it, not even by the gravest sin, not even by renouncing faith.