It means to multiply or abound. Usually we use it do describe an unorganised group, and depending on the context it can refer to a couple of different things.
To swarm about:
- Dicen que las ratas pululan por las alcantarillas de la ciudad.
They say that rats are swarming around the city sewers.
- Las hormigas pululaban en el edificio vacío por la cantidad de comida.
The ants multiplied in the empty building because of the amount of food.
To mill around:
- Era tarde pero mucha gente seguía pululando por las calles.
It was late but plenty of people were still milling around the streets.
This word of the day has been contributed by LAE Madrid, the leading Spanish academy in Madrid. Accredited by the Insitituto Cervantes, it offers Spanish courses for all levels and also has Spanish classes for kids and families.