1. What does it mean?
An abbreviation of 소소하지만 확실한 행복. It puts a name to modest, tangible pleasures: a quiet break, a favorite snack, or another ordinary moment that reliably makes a day better.
2. The nuance
The point is the happiness, not the purchase. It need not cost money or look impressive to anyone else. It also need not be a grand answer to life's problems.
3. Examples
“비 오는 날 창가에서 책 읽는 게 내 소확행이야.”
Reading by the window on a rainy day is one of my small everyday joys.
“오늘의 소확행은 퇴근길에 본 예쁜 노을.”
Today's little moment of happiness was the beautiful sunset on my way home.
Sources
Korea Policy Briefing: language and culture
References support the meaning and usage. Example sentences are written for NoCapWiki.