Short answers, pulled from the story.
A mole in espionage is an agent recruited before having any access to secret intelligence, who then builds a career inside a target organization over years or decades. Most spies, such as Aldrich Ames and Robert Hanssen, are recruited or volunteer after they are already in place; a mole, by contrast, is recruited first and infiltrates from outside.
The term was introduced to the public by British spy novelist John le Carré in his 1974 novel Tinker Tailor Soldier Spy. Le Carré said the word was actually used by the Soviet intelligence agency, the KGB, not coined by him. The term had also appeared as far back as 1626 in Sir Francis Bacon's Historie of the Reign of King Henry VII, though le Carré said he did not draw from that source.
The Cambridge Five were five upper-class British men recruited by the KGB as communist students at Cambridge University in the 1930s. They subsequently rose to high levels in various parts of the British government, making them among the most prominent examples of long-term moles in espionage history.
Because a mole's recruitment occurs in the remote past, long before they hold any position of access, there is no suspicious contact with foreign handlers to detect at the moment they join the target organization. Security clearance processes screen for visible signs of disloyalty, which a well-placed mole is trained to conceal from the outset.
Ideology, particularly sympathy toward world communism, was a major motivator for moles recruited in Western countries during the Cold War. During the 1920s through the 1940s, some Westerners became disaffected with their own governments and drawn to communist ideals without formally joining a party; these individuals were called fellow travellers.
James Angleton, who directed CIA counterintelligence between 1954 and 1975, was removed from his position in 1975 after reportedly accusing numerous prominent figures of being moles, including Secretary of State Henry Kissinger, Canadian Prime Ministers Lester Pearson and Pierre Trudeau, British Prime Minister Harold Wilson, and many members of Congress.