An onomatopoeic adverb meaning to stick closely or tightly (with no gap); also used for something coming to an abrupt stop (e.g. silence falling).
ガムが靴底にぺったりくっついて取れない。
The gum is stuck to the sole of my shoe and won't come off.
子どもは驚いてぺったりと黙り込んだ。
The child was so surprised that he fell completely silent.
ポスターを壁にぺったり貼っておいた。
I stuck the poster flat against the wall.