To Get Between
To Be Caught In
intransitive verb, godan verb
You already learned the word 挟む is for putting something between two things. 挟まる is the intransitive version — it's used to describe when something gets caught between two things with no direct object. So it means to get between or to be caught in.
You use 挟まる for situations like your jacket getting caught in a car door, food getting stuck between your teeth, or a memo being tucked between pages.
Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You already learned this reading when you learned 挟む, so you should be able to read this word as well.
アスパラガスが歯に挟まっています。
You have asparagus caught in your teeth.