No, this isn't where Illustrator is sent to a corner by itself for a time out! Isolation mode is one of the coolest features of Illustrator. It's been around since Illustrator CS2, but it's been tweaked a bit in each new version.
Simply put, isolation mode lets you "isolate" an object, group, or layer so that you can work on the object, group or layer without fear of accidentally changing the surrounding artwork. It's a great alternative to constantly ungrouping items or repeatedly locking and unlocking objects or layers.
Isolation mode is pretty simple to use. Just double-click on an object or a group with the Selection tool. You'll see the rest of the objects on the page dim, and a breadcrumb navigation path appear at the top of the active window.
Now you can edit the isolated object or group to your heart's content. When you're finished, either click on the breadcrumb navigation trail to back out of isolation mode, or just double-click somewhere outside of the isolated objects.
Many people don't realize that individual layers or sublayers can be isolated as well. Just select a layer or sublayer in the Layers panel, and choose Enter Isolation Mode from the Layers panel menu.