Placing media by eye is imprecise, and on a poster made of many sheets a small misalignment is visible across the whole wall. The canvas has three aids for this.
Snapping
As you drag media, it snaps to meaningful positions — the canvas centre, the canvas edges, and the edges and centres of other media. A guide line appears showing what it aligned to, and disappears when you let go.
Nothing is locked. Keep dragging and the media leaves the snap; the snap is a suggestion the drag prefers, not a constraint.
The tile grid
The canvas draws the boundaries of the sheets your poster will be cut into. This is not decoration — it is where the scissors go, and it is the single most useful thing on the canvas when you are placing content. It is on by default and can be toggled off from the poster header; turning it off also stops media snapping to the seams.
Use it to keep a face, a logo or a critical word away from a seam. Moving a subject two centimetres is free before you print, and impossible afterwards.
Rulers and your own guides
Turn on rulers and they appear along the top and left of the canvas. Drag from a ruler onto the canvas to drop a guide line at a fixed position, and media will snap to it like any other alignment target.
Custom guides are for the constraints the app cannot infer: the visible area inside a frame, a consistent margin on a series of posters, the fold line on a banner.
In the browser and desktop apps, G toggles rulers and guides together. See keyboard shortcuts.
None of this prints
Snapping guides, the tile grid, rulers and your own guides are editor aids. They are drawn on the canvas and never in the export.
What does print, if you turn it on, is the cut lines, scissor marks and page numbers — those are export options rather than canvas aids. See cut lines and scissor marks.