A newcomer is quite simply one who has recently joined the SCA or a similiar organization. The term newbie is slang for a newcomer.
In some areas, a newcomer may be formally defined (for instance as someone who has attended less than 3 events) and this may mean that they can get in to events at some kind of introductory rate.
Fortunately for most new people, it is quite possible to participate in the SCA on a small budget.
A joke runs that a newbie is "anyone who has been in the SCA less than 10 years"