Something that is soft and sweet by the taste and made from grains. Many of them is called bun in english but not all.
Cinnamon roll, Butter bun, Cardemumma bread, raising bun, braided loaf, etc.
Plain white bread in France or UK can be called pulla too. Kind of mocks it from not being enough heathly to be called bread. Also because people eat it with jam we think it more like pastry.
If you were just saying the type, you'd say "(haluan) pullaa, kiitos", where "pullaa" is in the partitive case because it's either an indeterminate amount of pulla pieces, or possibly it would just be considered the material of what you want to eat. If you want two buns, you could say "kaksi pullaa, kiitos". The partitive -a is also used for any discrete quantities other than 1. So "pulla, kiitos" is implicitly the same as asking for "yksi pulla, kiitos".
And as a sidenote, you've just learned 3 common situations where you need to use the partitive!