What is fixed base simulator?
A fixed base simulator is a flight training device with a grounded base (a fixed copy of an airplane flight deck). So, unlike a full motion simulator, it does not move while being operated. The purchase and operating costs are also much cheaper than a full flight simulator.
There are generic fixed base simulators, or simulators designed as a copy of a particular airplane type (e.g. B737 or A320). Simulators have to be certified to be used for pilot training. You can also find uncertified devices that are just used for experience (entertainment) flights for the public.
A fixed base simulator is especially suitable to train standard operating procedures (SOPs), checklist flow, aircraft systems and so on. To train flying skills, a full motion flight simulator (FFS) is required. For the reasons outlined above, FTDs are often used for MCC/JOC training or initial type rating.