OLED is an emissive display technology - which means that it emits its own light, in contrast to a reflective display - which uses an external light source - an ePaper display for example, or an LCD which is a display that blocks light from a backlight unit.
OLEDs are bright, and provide great image quality, but because they are emissive, when viewed under direct sunlight (or any strong light source) they have a readability problem.
In early AMOLED displays, sunlight readability was very pure. The 2008 Nokia N85 for example, one of the first products with AMOLED displays, behaved very poorly in direct sunlight, as can be seen in the image below:
However display makers (lead by Samsung Display, the leader in AMOLED production) soon upgrades their OLEDs to included better solutions and today OLEDs actually perform better than the best LCDs in direct sunlight.