This is the 7285th most frequent Russian word.
всероссийском
"Всероссийском" means "nationwide in Russia" or "all-Russian."
In this sentence, 'всероссийском' is used to indicate that the forum has participants or effects across all Russia.
На всероссийском форуме обсуждались важные вопросы.
Important issues were discussed at the all-Russian forum.
Here, 'всероссийском' is used to denote the nationwide scope of the competition.
Всероссийском конкурсе приняли участие лучшие проекты.
The best projects participated in the all-Russian competition.
'Всероссийском' here emphasizes that the level of the event encompasses all of Russia.