QUICK ANSWER
"Qué pasó" is a form of "qué pasó", a phrase which is often translated as "what happened". "Pero" is a conjunction which is often translated as "but". Learn more about the difference between "pero" and "qué pasó" below.
pero(
peh
-
roh
)A conjunction is a word that connects words, phrases, clauses, or sentences (e.g., The cat and the dog slept.).
a. but
Te quiero comprar algo, pero no tengo dinero.I want to buy you something but I don't have any money.
A masculine noun is used with masculine articles and adjectives (e.g., el hombre guapo, el sol amarillo).
qué pasó(
keh
pah
-
soh
)A phrase is a group of words commonly used together (e.g., once upon a time).
1. (general)
a. what happened
¿Y? ¡Cuéntame! ¡Muero por saber qué pasó!So? Tell me! I'm anxious to hear what happened!