“Plums” (ciruelas) you may know, and these grow on plum trees. In English cuisine there are “plum puddings”, and plum tomatoes (tomate pera). It is also a colour (color ciruela).
A “plum job” is a very good job, perhaps as a result of the salary and benefits. Perhaps, in Spanish you would say “un puesto de trabajo fantástico, o un chollo”.
There is also an idiom (modismo) using “plum”. Sometimes, a mortal speaks as if he had a “plum in his mouth”, probably in an artificial or superior way. Perhaps in Spanish you would say the person “habla en una manera engolada”.