This means simply that the department guarantees that they will receive a full RA or TA for their first semester. Note, however, that each of these students still has to accept a particular position from among the offers that are made to them by individual faculty members. If they wish to work with particular faculty, they should feel free to contact them and enquire about the availability of positions, just like the students who have not received guaranteed offers. Students seeking research assistantships should be aware that many, if not most, commitments are not made by April 15. Positions become available through the spring and summer, depending on when the research grants come in, and a significant fraction of new research assistantship offers are made after the beginning of the fall term.
Most graduate students end up with funding for their first term.
Teaching assistantships are relatively few in number, and usually offered to students who are already at MIT and known to the departmental faculty.