This is the 6100th most frequent Russian word.
срочно
Urgently.
Here, 'срочно' is an adverb used to describe the immediacy of the action required for sending the letter.
Мне срочно нужно отправить это письмо.
I urgently need to send this letter.
In this sentence, 'срочное' serves as an adjective modifying the noun 'дело', indicating the importance and urgency of the matter.
Он сказал, что это срочное дело.
He said that this is an urgent matter.
'срочно' here again functions as an adverb, emphasizing the need for immediate communication.