An adverb meaning "that much" or "so" (to that extent); often used in negative sentences to mean "not so much."
そんなに急がなくても大丈夫ですよ。
You don't need to hurry that much; it's okay.
彼はそんなに背が高くない。
He isn't that tall.
そんなに褒めないでください。照れます。
Don't praise me so much. I'm embarrassed.