Onload uses fds internally that are not visible to the application. This can cause problems for applications that make assumptions about their use of the fd space, for example by doing dup2/3 onto a specific file descriptor. If this is done on an fd that is internally used by Onload than an error of the form 'citp_ep_dup3(29, 3): target is reserved, see EF_ONLOAD_FD_BASE' will be generated.
This option specifies a base file descriptor value, that Onload should try to make its internal file descriptors greater than or equal to. This allows the application to direct Onload to a part of the fd space that it is not expecting to explicitly use.