An SRB file is an archive used by Sonic Robo Blast 2, a fan-made, open source Sonic the Hedgehog game. It contains most of the resources used by the game, such as textures, patches, sprites, palettes, and graphics files.

You most likely will not see the SRB file since it is used for storing resources and you will not directly open it in the game. However, if you are looking to modify gameplay, you can open the SRB file in SLADE and XWE. When starting the Sonic Robo Blast 2 game, the SRB file is the first game resource file loaded. It is the main game resource file and is saved in the .WAD (Where's All the Data?) format, which is a data library used by games that run on the Doom engine. Sonic Robo Blast 2 was built using a variation of the Doom Legacy port of Doom.