Looks like it got "sunburned". When you have an inside plant that gets put outside, it can happen - too much light too soon. Unfortunately, plants don't heal a sunburn the way we do - you end up just pruning off the "burned" parts.

To help avoid this problem in the future, try just setting the plant out for a couple of hours per day, gradually increasing the time out over couple of weeks. Try to make sure it stays a shady area for a while before moving it into a sunnier area.