Matthew 28:19 Jesus instructs us to "Therefore go and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit.."
The Standard definition of a disciple is someone who adheres to the teachings of another. The term refers to someone taking up the ways of another.
In saying that, these shirts declare two things. That one, we are a disciple of Christ and two we are to go out and make disciples. We as a disciple of Christ will adhere to the teachings of Christ, follow His ways, deny ourselves and take up our cross. We pray also that we will go out and make disciples of all nations just as he commands in Matthew 28:19.