An adverb meaning excessively or indiscriminately; often used as 'やたらと'.
今日はやたら寒いから厚手のコートを着て行ったほうがいい。
It's unusually cold today, so you should wear a thick coat.
彼はやたらと人の話を遮る癖がある。
He has a habit of interrupting people all the time.
やたらと写真を撮って思い出を残した。
I took a ton of photos and left myself with memories.
As a quasi-adjective, indicating something indiscriminate or excessive—usually carries a negative nuance.
やたらな要求は却下された。
The excessive (and unreasonable) demands were rejected.
やたらな投資は危険だ。
Indiscriminate investment is dangerous.