How to | Insert a File Path
One way of telling the Wolfram System where to find a file or directory is to type in the full path.
For example, import the size of this image by typing in the full file path:

https://wolfram.com/xid/0bcnb0drzwcopeukr8wbz7rxi-nyxrnp

The parts of this file path are separated by the \\ character, which is used in strings to represent the \ character, the file separator on the Windows operating system.
If you do not know the full file path, you can use the Insert ▶ File Path menu item. This lets you use your computer's file system browser to select a file:

When using this menu item, you must select an individual file. The Wolfram System will not let you select a folder. To insert a folder path, select a file in that folder and manually delete the file name from the path.
Alternatively, you can use DirectoryName with the file name:

https://wolfram.com/xid/0bcnb0drzwcopeukr8wbz7rxi-o7oa6n

You can also insert the path of a folder or a file by simply using drag and drop in combination with a keyboard shortcut.
In Mac OS X, first navigate to the file or folder. Click and drag it over a notebook, and then hold down the key when releasing the mouse button:

In Windows, do the same thing except hold down the keys when releasing the mouse button. The cursor will change to indicate that a path will be inserted when you release the mouse button:

In Linux, the keyboard shortcut for inserting a file or folder path with drag and drop is . However, depending on your file manager program, you may not see a change in the cursor. Even if the cursor does not change, holding
while dragging and dropping a file or folder will still insert the correct path.
In Windows and Linux, inserting a file or folder path with drag and drop will not work in versions prior to Mathematica 8.
To learn about programmatically constructing file and directory paths, see How to: Input and Construct File Names in the Wolfram Language.
For information on identifying and changing the directories that the Wolfram System automatically uses when looking for files, see How to: Locate and Use Files.