Bleed is the edge of a page which is going to be cut off. If it is necessary for the print to cover the printed matter all the way to the final edge, the printed design must continue over the further trim. This is necessary to avoid having a white border on some pages which can happen due to printing and trimming inaccuracies.
Trim box (finished size) shows where the final printed matter is cut. The trim box of the final sheet is the area defined by the upper-left corner of the first position and the lower-right corner of the last position. It contains all the positions of the sheet.
Untrimmed size, Bleed box of the resulting sheet is larger or equal to the trim box. It can be used for bookmaking to cut the final sheet, process it (fold, sew, glue etc.) and then cut to the finished size. It can be set to pages bleed or customized as needed.
Imposition type is explained here in detail.
Imposition margin is the area where various marks (trim marks, registration marks, sheet descriptions…) and bleeds are placed. Its size can either be calculated automatically from page bleed and marks or it can be set manually. If the automatic sheet size is selected the sheet is set exactly to the imposition margin. If the sheet size is set manually the real sheet border (the distance between the trim box and the sheet end) can be bigger based on the size and sheet alignment.
Gaps between rows and columns are necessary if the final printed matter needs to be covered in print to its edge. When the gaps are zeroed and the graphical design is different on each page or in a case of a different background color the edges of some pages can be covered by the design from another page which is caused by inaccurate print and trimming. See Bleed.
Sheet, sheet size is the sheet size of the resulting PDF. If the imposition cannot fit the area it is highlighted.
Imposition size is the part of the sheet which is covered by the imposition (including the edges with crosses and marks).
Position is a part of the imposition where single pages are placed. These positions are organized into rows and columns same way as in tables.
2-UP (or more) the same page is the case where one or more pages are placed on a single sheet multiple times. It can either be achieved by setting page repetition or by placing the desired page on multiple positions (for example by dragging from the palette Pages for imposition). Please, do not combine these two methods! If you are using a booklet imposition page repetition cannot be used. If a group of pages need to be placed on a sheet multiple times (for example for a booklet the first and the last page are placed next to each other) the 2-UP can be realized by choosing Menu->Edit->Duplicate imposition below (side by side).