Alex, I think Victoria explained it fairly well. But I'll add a few sentences. Sometimes in Lr, you want to do something to a single photo. Sometimes you want to make the same changes to multiple photos. Lr provides several methods of applying the same settings to multiple photos.
Let's go to Develop module with the filmstrip visible. Select three adjacent photos in the filmstrip. Note that one has a light gray frame, two have a medium gray frame, and all of the unselected images have a dark gray frame. The lightest gray frame is termed the 'most selected' or 'active' photo. Note that you can change which photo is the most selected, without disturbing the overall selection by single clicking within the actual image area of a filmstrip thumbnail. Make an adjustment or two to the selected photo. Then click on the Sync... button at bottom of Lower Right Hand Panel. You'll open a dialog asking what adjustments you'd like to synchronize from the most selected to the remaining selected. Click Sychronize, and the changes will be applied to the rest of the selected photos. Note 'synchronize' is not exactly the right word, because the transfer only goes one way, from the most selected to the others, and not both ways, which is the usual meaning of synchronize. Take it one step further, same situation in Develop, but before you make any changes to the most selected, hold down Ctrl/Cmd, and note that the Sync... button becomes 'Auto-Sync' . Press that. Now any changes you make to the most selected image are automatically applied to the remaining selected files as well. ***Remember to turn off Auto-Sync, or chaos and confusion will rapidly ensue.
This somewhat elaborate rigmarole is designed to allow you to work with full screen image detail for a source image, while allowing simultaneous manipulation of an unlimited number of target images. If you use Photoshop or similar, think how complicated this would be to open multiple images, run some sort of filter routine, save and close, perhaps hundreds of images. It provides a highly visual user interface into a farily complex batch model.
The same paradigm appears repeatedly in Lr, for instance, applying metadata to multiple images works in much the same way in Library Module.
I see my 'few sentences' have run overtime. I'm hopeful that between my post and Vic's, this is helping you see the light to a massive improvement in your productivity.