A detailed simile (mufaṣṣal) explicitly states the respect in which the comparison holds. A summary simile (mujmal) leaves it unstated, to be inferred from context, convention, or imagery.
Detailed: “Zayd is like a lion in courage.” Courage is named.
Summary: “Zayd is like a lion.” Courage may be understood, but is not stated.
Mujmal does not necessarily mean unclear or weak. A highly familiar comparator can make omission both intelligible and powerful.