To Solidify, To Become Firm, To Become Certain
Intransitive verb and Godan verb
かたまる
みんな何でそんなすみっこに固まってるの？
Why are you guys packed into the corner like this?
ようやくトーフグをやめる決心が固まりました。
My decision to quit Tofugu finally solidified.
そろそろプリンが固まる頃かしら。
I think the pudding will finally harden soon.
You've seen how 固める means "to make hard." This word means that something hardened by itself, it means to harden or to solidify.
Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You learned this reading already so you should be able to read it!