Learn Spanish (Latin America): Me gusta viajar, sobre todo en verano.

Sobre todo literally means 'above all' and is the most natural way to say 'especially' in Spanish. Similar to how incluso (even) highlights an extra or surprising addition, sobre todo acts as a focusing tool to highlight the most prominent example from a group. You could also use especialmente, but sobre todo is incredibly common in everyday speech.

Wait, doesn't 'travelling' translate to viajando? Why use viajar?

In English, we often use the '-ing' form for activities we like (I like travelling). In Spanish, when a verb acts as a noun like this, you must always use the infinitive form (viajar). Never use the '-ndo' form (viajando) after verbs like gustar.

Why is it en verano instead of en el verano?

When talking about seasons generally after the preposition en, Spanish speakers frequently drop the article (el). Saying en el verano is also completely correct and very common, but en verano is just a slightly shorter, idiomatic alternative.

Is sobre todo always written as two separate words?