Grouping is a wonderful thing when you need to treat a collection of objects as a whole. If you simply click any object in a group with the Selector tool, the entire group gets selected. However, quite often you want to select and edit an object inside a group without ungrouping it. This is possible by 
-clicking an object in a group.
-clicking completely ignores any grouping, no matter how many levels deep it is. For example, if object A is a member of group B which, in turn, is a member of top-level group C, then -clicking A will select A, cutting right through both levels of grouping. There’s no way to select group B by -clicking; it will always select only the lowest-level nongroup object.
The only way to select group B, which is inside group C, is by entering group C (see 4.6.1 Layer Hierarchy). Entering C makes it a temporary layer. Once in that layer, you can select B by simply clicking.
-click can be combined with 
(select under). Logically, 
-click does the same thing as -click, except that it disregards any groupings and browses through the z-order stack of objects at the click point as if they were all ungrouped.
Similarly, -click can be combined with 
to add an object to selection or remove it from selection. Finally, you can 
-click, which means “Add to selection the topmost nonselected object in the z-order stack at this point, ignoring grouping; if all objects at this point are selected, deselect the topmost one.”
The only limitation to the power of selecting by clicking with various modifiers is this: You cannot have both an object and a group which contains that object selected at the same time. So, for example, if you -click an object inside a group and then -click (without !) another object of the same group, thus trying to add the group to the selection, the group becomes selected, but the first selected object is deselected. A situation where both a group and an object inside that group are selected at the same time would lead to various logical impasses, so Inkscape does its best to prevent this from happening.