Learn Spanish (Spain): Se cree que los fantasmas aparecen en invierno. - It is believed that ghosts appear in winter.

Se cree uses the impersonal se. It translates to 'it is believed' or 'people believe'. We use it when we aren't talking about a specific person believing something, but rather a general fact or widespread rumor. If we just said Cree, it would mean 'He believes' or 'She believes'.

English just says 'ghosts appear', so why is there a los before fantasmas?

In Spanish, when you talk about things in a general sense, you almost always need the definite article (el, la, los, or las). Since this sentence is talking about ghosts in general, we must say los fantasmas.

Why is aparecen plural when se cree is singular?

Aparecen (they appear) is plural because its subject is los fantasmas (the ghosts). The first part of the sentence, Se cree (it is believed), is a separate part of the sentence. The ghosts are the ones doing the appearing, so the verb aparecer must match them in the plural form.

Do we always say en invierno without the word for 'the' (el)?

When using the preposition en (in) with seasons like invierno (winter), Spanish usually drops the article. So 'in winter' is simply en invierno. If you were talking about a specific winter, you would include it, like en el invierno pasado (in the past winter).