Solid
Tough
い adjective
This is a single kanji with an い on the end, meaning you know it's probably an adjective. What's the adjective form of solid? It's solid or tough. This "solid" and "tough" is a figurative version of the word.
かたい
The reading is the same as the one you learned with the kanji.
レシピには、美味しくてしっとりしたクッキーって書いてあったのに、出来上がったクッキーはすっごく堅かったんだよね。
The recipe calls them delicious, moist cookies, but they turned out to be really hard.