Is it possible my neighbor could hack in and read my files on my Macbook?
Whether a neighbor hacks into your network/computer is hard to say without detailled analysis. What you can do to reduce the risk is to 


Make sure your WiFi is set to WPA2
Change the password on your WiFi to something which is not/not easy to guess
Change the passwords on all the accounts on your computer
Check whether there are accounts visible in System Preferences->Users &amp; Groups which don't belong to you


This will not prevent a dedicated attacker from finding a way in of course, but might be enough to keep sneaky neighbors out.