Several different terms are used in this help file so to familiarize you with them we've listed them here:
Canvas: The overall background of your collage. Cells are displayed on your canvas.
Cell: A cell is an opening on the canvas. Photos, captions and calendars can be added to a cell.
Photo: A photo is inserted into a cell.
Bound: In previous versions your poster cells could not be moved around on the canvas. Bound posters allow you to quickly create posters with many cells and have everything line up exactly with each other cell. To retain the features of the grid style poster you have to keep your poster bound.
UnBound: An unbound poster allows you to move cells around on the background. You are allow to create freeform collages with an unbound poster.
Locked: If your poster is unbound then you can move your cells around. At some point you will have finished the layout that you want to use and will want to lock the poster so that these cells do not accidentally get repositioned or resized. If you should desire to move your cells around you can unlock your poster and make your adjustments. After you are done then you can lock the poster again.