Development Mode temporarily allows you to temporarily suspend CloudFlare's edge caching and minification features.
You will find the Development Mode option by clicking on the settings gear icon next to your website name under your CloudFlare account:
Once entered, development mode will last for 3 hours and then automatically toggle off, assuming you don't manually turn it off earlier.
NOTE: Development Mode does not clear the edge cache for your site - it simply suspends the use of it while Development Mode is enabled. If you need to specifically clear the cache you should use the Single File Purge or Full Cache Purge functions.