Direct sunlight is pretty literally; direct sunlight onto the patch. So when for instance sticking it on an upperarm and not wearing a shirt with sleeve covering the patch when out and about a Florida sunny day. Or when wearing a very flimsy shirt (you'll know the one, the type you also could get sunburnt in when wearing them, not using sunscreen underneath, having sensitive skin and then spending a day out in a blistering summer sun) over it and full sun on it for hours. So it's nothing like some might know from antibiotics where one would have to stay clear from direct sunexposure themselves, with this it's nothing more than the patch. You can kinda compare it to having guidelines how to store medication in pill forms at room temperature or certain fluid medication in a fridge to keep it from spoiling, dosage/working altering. Don't know about all the fentanyl brand patches (there are more than one brand) so you'll want to find out about yours but there are quite a few that come with info to be aware of this.
I wear my patch on my ribcage, mostly side. This has the bonus of your upperarms protecting it very easily long periods of time already since than is a natural posture, is a location that is covered with clothing about almost when out in the sun (and thus public, most of times, esp. at WDW). Tend to wear cotton shirts/tops/whatever upperbody clothing in Florida. Fit cut. Sometimes the side of a bra might cover (part of) my patch if on that height, sometimes not. Have been alternating after 2 days for about 2 years orso now, but also back when it was still 3 days no problems in Florida sunshine with my way of clothing, where I put them and preventing the direct-on-patch sunlight.
Had to learn it the hard way though, thans to info not being that clear in the infobooklet (much clearer discussed in professional info than consumer info, or at least here it is) and me not taking it that seriously reckoning Dutch sun in spring couldn't be all that bad. Since then I always keep a layer of clothes covering it, many times also arm in the way as said and never been a problem since incl. in even harsher sun conditions than Florida.