Learn English – the meaning and structure of the following sentence

I assume this is taken from a technical piece of writing, such as the documentation for a program, so I will start by saying that you will not run into sentences this convoluted elsewhere. In documentation, people often cut down on words in order to save space. A fuller version of this sentence might be:

RLIMIT_NOFILE is defined as one more than the maximum number of file descriptors that a process may hold open.

On to breaking down the meaning of the sentence.

the maximum number of file descriptors: A process can hold open a limited number of file descriptors. There is some maximum number (an upper bound).

one greater than the maximum number of file descriptors: As stated above, there is an upper bound to the number of file descriptors a process may hold open. "One greater" means "one more", so you are adding one to the upper bound.

Specifies one greater than the maximum number of file descriptors that a process may hold open.: The number RLIMIT_NOFILE gives (that is, specifies) the upper bound + 1 of file descriptors that a process can hold open.