This is the 1397th most frequent Korean word.
에는
"에는" is a postposition in Korean that means "in," "on," or "for" when indicating a subject, time, or condition in context.
In this sentence, '에는' is used to indicate the topic or context ('for swimming') for which the statement ('a lot of energy is needed') applies.
수영에는 많은 에너지가 필요합니다.
Swimming requires a lot of energy.
Here, '에는' specifies the timeframe or occasion ('on the weekend') associated with the subsequent action ('went for a walk').
주말에는 친구들과 산책을 갔어요.
On the weekend, I went for a walk with my friends.
The usage of '에는' in this sentence points out the topic ('happiness') concerning which the statement ('small things are important') is being made.