“Mole” (topo) you may know as the small black animal that digs tunnels under the ground, perhaps under your lawn. A mole is also a dark or black spot (lunar) on your skin.
Sailors also know about moles (malecón, espigón, rompeolas). These are constructions on the coast that act as a breakwater (rompeolas), that is, preventing big waves getting close to a harbour.
There is another use. Have you ever read “spy novels” (novelas de espías)? Probably. In these novels there is often a “mole”, a person who reveals secrets (without permission) to another organisation.
Let´s imagine that you are a member of Mr. Sanchez´s cabinet (gabinete ministerial). The cabinet meetings are normally “confidential”. You sometimes secretly (a escondidas) give information about these meetings to the press or another organisation. You are, therefore, a “mole”.