What an IQ score is
IQ is a score from a standardized test of reasoning and related abilities. The number only has meaning relative to the people the test was tried on.
Why norms matter
A professional test is first given to a large sample of people chosen to represent a population. Scores are then set so that the middle of that group is 100, and most modern tests use a standard deviation of 15. Without this step a number looks like an IQ but does not compare you with anyone.
What an online test can do
It can give you interesting puzzles, a rough sense of how you handled them, and practice. It usually runs without supervision, on different screens, and possibly with distractions or help, so its number should be treated lightly.
What it cannot do
It cannot diagnose anything or support decisions about school, work or health. If you need an assessment for one of those, a licensed psychologist can give you a properly administered test.
What IQ does not capture
A score is one narrow measure. It can be influenced by education, practice, health, and how you feel on the day, and it does not sum up a person.