An xlsb file is a type of spreadsheet. Normally, a spreadsheet is saved in XML format. However, with an xlsb, the information is saved in binary format. This means that the file will be slightly larger when it is saved, but the information can be accessed and manipulated much faster. For very large spreadsheets with many calculations, this is a reasonable trade off.
To open an xlsb file, you will need a spreadsheet program. One of the best programs available is Open Office, and even better, it is free. Open Office is released under a “free software” license. What this mean to you is that you will never have to pay royalties or service fees for using Open office, even if you have hundreds of computers. The spreadsheet part of Open Office, called Calc, is able to handle anything that you would need out of a spreadsheet. With Open Office, you never have to fear that a software vendor will release a new version with a new file format, forcing you to upgrade all of your software. This, obviously is an unnecessary expense if you use Open Office. Open Office is able to open xlsb files at not cost, so why not try it out?