Not all of the channels in the 5 Ghz band are useable all of the time. If you are near an airport then some of the 5 Ghz channels are not available to you. DFS stands for Dynamic Frequency Selection so basically you have that feature turned on and your APs will listen for Radar or first responders using those 'shared' frequencies and not use those channels if they are being used.
Check this out.
The 5GHz channels don't overlap, BUT if you use channel bonding for 802.11n or 802.11ac to bond more channels to a 40 or 80 MHz channel you'd need to make sure you don't overlap.
In the latest controller version the GUI also show which of your 5GHz channels are non-DFS - just choose custom channel plan and you get the list.
The below screenshot is for AP country set to Austria ...