Minecraft wiki entry for Target_selectors, associated text: [dx=<value>,dy=<value>,dz=<value>] — Filter target selection based on their x-difference, y-difference, and z-difference from some point, as measured by entities' hitboxes in Java Edition or by their feet in Bedrock Edition. Cannot duplicate any one of these three arguments.
This can be interpreted as creating a rectangular volume defined by an initial position (<x>,<y>,<z>) and diagonal vector (<dx>,<dy>,<dz>), then selecting all entities whose hitboxes are at least partially contained by that volume in Java Edition, or whose feet are within that volume in Bedrock Edition. If the positional arguments are left out, the selection is interpreted as originating from the position of the command's execution. Any values are allowed, including signed and fractional numbers.
Note that dx,dy,dz specify signed differences from the given coordinate. They do not specify a separate coordinate, nor do they extend in both the positive and negative directions.
Additionally, when any volume argument is present, the rest are assumed to equal zero unless otherwise defined.