The standard archival designation is organized as archive/fond/inventory/file. Thus the example you gave works out to
KRA - the Kaunas regional archive (other archival abbreviations frequently encountered in Lithuanian records are LVIA, the state historical archive, and LCVA, the central state archive, both located in Vilnius)
I-210 - the number of the fond at KRA
1 - the number of the inventory (ie, a section) within that fond
439 - the number of the file within that inventory
Many Lithuanian records are online at familysearch.org, but not this specific file as yet. You would have to purchase a copy of your records from KRA (kaunas@...). The records would be in Russian as the area in question (Rokiskis) was part of the Russian Empire at that time.
Records Acquisition & Translation coordinator, LitvakSIG