When woring in an assembly that contains components and selecting and moving stuf using the viewport it is very easy to move a body within a component and that is usually not what you want!
To avoid this you can go into the slection filters and turn the Bodies selection filters off, so then when you sleect geometry in the vieport you really slecting a component and not a body. I bet this confuses many users coming from traditional mechanical engineeering softwares e.g. Solid Works, Solid Edge, Geomaic Design ad can lead to quite some frustration.
Here is a screencast that hopefully explains that a bit better.