This is the 4417th most frequent Japanese word.
固
"固" means "hard," "solid," or "firm."
固い is used here as an adjective suggesting physical hardness.
これらのパンはかなり固いです。
These breads are quite hard.
固める is used here as a verb meaning to strengthen or solidify something.
彼は自分の決意を固めた。
He solidified his resolve.
固すぎる implies that something is excessively rigid or inflexible.