Thank you for the opportunity to help you with your question!
Fixed inputs are those that takes time to adjust when a company grows. Capital is one of them. When a firm expands, it takes longer to acquire new investments. In other terms, it means that the size of the plant or factory is unlikely to change within that period of time.
There is also technology. Technological changes depend on Research & Development outcomes or new discovery. Moreover, these technological advances have to be tested and approved for consumer use. It takes quite some time for these technological advances to be available for use.
This period of time where some inputs are fixed is called the short run.
Please let me know if you need any clarification. I'm always happy to answer your questions.