"Unlockable bonuses" refers to additional perks, rewards or features that become available once certain conditions are met or achievements are earned in a game or other activity. Full definition
There's a nice variety of unlockable bonus material here, including video interviews with Sega development staff and several of Sega's arcade games (Zaxxon, Zektor, Altered Beast, Tac / Scan, and Future Spy). (gamesetwatch.com)
The Orochi Saga will include unlockable bonus material as well as a Color Edit Mode to personalize favorite characters. (thatvideogameblog.com)
In addition to the Free Play modes, there are unlockable bonus levels for each Episode once you complete all six chapters and have enough Gold Bricks. (snackbar-games.com)