たちまち is an adverb in that is used to convey the feeling of something happening 'suddenly', or 'in a flash'. The primary nuance with
たちまち compared to other similar structures is that
たちまち indicates that something happened spontaneously, and was not able to be controlled.
In many cases,
たちまち will indicate something entering a new, ongoing state, or something being completed.
たちまち will often appear directly before the verb phrase that it is describing as being the new state.
あのレコード会社（がいしゃ）に所属（しょぞく）するアーティストたちはたちまち有名（ゆうめい）になる。
Artists who belong to that record company quickly become famous.
私（わたし）と田中（たなか）さんはたちまち仲良（なかよ）くなった。
Tanaka-san and I immediately became friends.
冷蔵庫（れいぞうこ）に入（はい）れておいたシナモンロールはたちまち妻（つま）の胃袋（いぶくろ）へと消（き）えた。
The cinnamon rolls I had put in the fridge quickly disappeared into my wife's stomach.
Fun-fact - たちまち originally comes from the kanji structure 忽（たちま）ち, with the kanji itself meaning 'without thought'. This nuance is kept with たちまち, as all actions associated with the word indicate a lack of planning or thought before happening. Despite this, the kanji itself is rarely used in modern writing.