The Difficulty Target is the number that will be used by an opposing party trying to counter or resist your spell. Thus the higher your level, the higher the DT for the spell, and the more difficult it is to counter the spell.
These spells come from the SRD 3.* system. All of Microlite20 is from that source. Unless the spell calls for a test of some sort they are automatically a success... you just have to pay the cost in HP. The difficulty test comes into play occasionally when you are casting against another magic user who can use a counter-spell against your spell, and monsters that have a natural defense against magic.