Short for attention-deficit hyperactivity disorder. At a glance, a person with ADHD may appear to others as childish and should just "grow up" and do their duties. The person with ADHD may try very hard to do so, but instead finds themselves unable to operate as others expect.
What is it like to have ADHD?
ADHD can be Inattentive, Hyperactive-impulsive, or Combined. This can be read up in more accurate detail on a more reliable source such as the CDC page. ADHD is a serious condition and you shouldn't rely wholly on social media sources for this.
That said, I realised I both had ADHD and that ADHD was causing me trouble when I learned I was experiencing ADHD procrastination.
ADHD is permanent, and while treatment for its negative aspects can be effective, there is 100% regression rate on symptoms - they are part of your character. That said, the negative side effects of ADHD can be handled in a variety of ways.