Admission
Entrance, Imprisonment, Internment
noun, verbal noun
You enter a place. So, you have been given (or forced) admission into this place. You have been given entrance. For example, "admission" into a prison is something you could gain by not doing your WaniKani reviews in certain Crabigator ruled countries.
入所 refers to the act of entering facilities or institutions, commonly those with names ending in 所, such as 研究所 (research institutes).
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
私のおばあちゃんは、三月にグループホームに入所しました。
My grandma joined the group home in March.
今日はモデルエージェンシーの入所テストでした。
I had an entrance test for a model agency today.
入所の前に必ずルールをチェックしておいて下さい。
Be sure to check the rules before your admission.