Admission
Entrance, Imprisonment, Internment
noun, verbal noun
You enter a facility. So this is your admission or entrance into that facility.
入所 is for entering facilities or institutions, often those with names ending in 所, like a 研究所 (research institute). It's also used in prison contexts. You might gain "admission" to prison for not doing your WaniKani reviews, for example, at least in countries ruled by Crabigators...
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.