It doesn't always mean that someone has a confirmed diagnosis of a disability, and there is no requirement for anyone to share that information, wearing one indicates that extra support, patience or understanding is needed. The emphasis above all is that of kindness.
The Sunflower Lanyard Scheme started in 2016 and is now recognised worldwide by many organisations as a way to facilitate a conversation about how to support someone in the right way. It is not mandatory to wear the lanyard if you have a disability...it's something that a disabled has chosen to wear to indicate they need support.
There are cases where the lanyard will come with an accompanying badge which tells people what support is needed, but its important to know this is not always the case.