From Bulbapedia, the community-driven Pokémon encyclopedia.
Dream Mist (Japanese ゆめのけむり Dream Smoke) is an item introduced in Pokémon Black and White. It has a strong association with Munna and Musharna.
In the games
Dream Mist is unable to be obtained by the player in Black and White, but is an important part of the storyline. After obtaining the Trio Badge, the player is asked to fetch some Dream Mist from the Dreamyard by Fennel, a scientist. However, Team Plasma also wish to obtain Dream Mist, and must be defeated before the Dream Mist can be recovered. After they are scared off by a wild Musharna, Fennel is able to recover her Dream Mist.
Back at her laboratory, she will reward the player for their efforts with a C-Gear. Without having undertaken this quest, the player is unable to leave Striaton City.
In the anime
Dream Mist portraying a dream
The Dream Mist appeared in the anime in Dreams by the Yard Full!. The Musharna living in the Dreamyard sent it out in huge clouds, causing Dream Mist to fall over Striaton City and put Pokémon to sleep. One of the victims was Iris's Axew, but Fennel's Munna was able to heal it by inhaling the Dream Mist that covered Axew. Munna's floral pattern then turned pink and a puff of mist came out of Munna's nose revealing Axew's dream of evolving into a Haxorus.
In the end, Fennel and her Pokémon were able to stop the Dream Mist from putting all the Pokémon to sleep.