1. What does it mean?
선팔 combines 선-, meaning ahead or first, with a clipped 팔로우. It describes initiating the follow before the other account follows you back.
Also written or said as seonpal
2. The nuance
선팔했다고 맞팔 is not an obligation: the phrase describes the order of actions, not a promise of reciprocity. It also says nothing about how close the people are.
3. Examples
“그림이 마음에 들어서 내가 먼저 선팔했어.”
I liked their artwork, so I followed them first.
“소개 글에 선팔 괜찮다고 적어 뒀어.”
I wrote in my introduction that people are welcome to follow first.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.