Its because you put the plant straight outside and the plant is not used to the harsh light outside so it gets 'sunburned'. To avoid this in the future you should harden it off by moving it out gradually over about a week:
- day: 2-3 hours in part shadow
- day: 2-3 hours in the sun
- 4-5 hours in the sun
- 6-7 hours outside
- the whole day out but in at night
- the whole day out but in at night
- Now it can be outside all the time.
It does not have to be exactly like this, but the idea is you let the plant get used to the environment outside slowly. I know it can be difficult to do this while having a full time job so you can make variations where you leave it longer in the evening where the light is less harsh and stretch it out over a longer period than a week. or you can put fleece over it and leave it out a bit longer as well.
About the plant where the damage is done. It will set new leaves to replace the damaged ones, don't worry :)