To Be Passionate About, To Be Into, To Grow Stiff
intransitive verb, godan verb
This word consists of kanji with hiragana attached. Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means congeal or freeze, so the verb version is to grow stiff. Remember 肩こり (shoulder stiffness)? This is where that こり part comes from. 凝る also means to be passionate about something. Think of it this way: your passions are freezing onto one thing, so you're going to be into that thing.
こる
Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You didn't learn that reading with this kanji, so here's a mnemonic to help you: What have all your friends started to be passionate about? Their 子s (こ). They all keep having kids and now that's all they seem to care about. Damn kids.
デスクワークのせいで肩や首が凝るのは仕方がない。
It's inevitable for your shoulders and neck to grow stiff from desk work.
最近、俳句に凝っているんですよ。
I've been into haiku these days.
妻の料理の盛り付けは、いつも凝っています。
My wife's plating of food is always elaborate.