Substances are real entities that have existence outsied the mind; i.e. your car, your Mon and Dad, the local Grocery store, your dog and cat, etc. You have ideas of these things too but you have them in a universal way, like the universal Mom or the universal dog, etc. These Universal ideas are " beings of the mind, " they do not have actual existence outside the mind.
Substance is understood at two levels. Second Substance would be Jesus as He walked among the Apostoles. The Christ in the Sacrament is just exactly like that, it is Jesus as He walked among the Apostles, except His Body is now Glorified and is present in the Sacrament according to the manner of the Sacrament. That is, His presence, by His will is made conformable to the necessary conditions of the Species ( the host and wine ), whose substance is changed into His Body and Blood.
First Substance really does not have anything to do with the Sacrament, but it is the reality of a thing in which all the accidents inhere. You cannot see First Substance, what you see is Second Substance, which is really noting but the total conglomerate of a Substances accidents - arms, legs, color, hair eyes, shape, weight, etc.
Linus2nd