You seem to be asking about what to call a block of text that appears in the space between a section heading and the first in a series of subordinate section subheads. At the various publishing houses where I have worked, the term I have used (and heard used) for such text is "section intro"; if there is a single word for it, I've never heard it.
The rationale for calling it a "section intro" is that it is only a small part of the full section that it introduces. The full section comprises the main section heading (in your example, "1 Introduction"), the section intro text, all of the hierarchically subordinate subheads (in your example, "1.1 Motivation" and "1.2 Objectives"), and all running text that appears beneath each subhead. The signal to readers that they have reached the end of a full section (that is, the section intro plus all subsections (including subheads) is the appearance of the next section heading (e.g., "2 Making Sense of Section Intros").
Section intro text is an extremely common element in publishing because many publishers require that there be at least some running text between any section heading or subhead and a following subordinate subhead or sub-subhead. This is not a universal rule, however. The Chicago Manual of Style, sixteenth edition (2919), for example, states, at 1.54 (Subhead levels and placement), "A lower-level subhead may follow an upper-level subhead with no intervening text." Still, in my experience, many publishing houses frequently break with Chicago on this point.