I do believe it exists. However I think it's very easy to misinterpret and often the books trying to explain it make it seem as easy as just "think positive" as people have said if it was as simple as that we would have no problems in the world.
It's a universal law that is taught about in various esoteric groups such as rosacrucianism or freemasonry
It pretty much fits with Jesus teachings in the bible "ask and believe that you have received" "as a man thinks in his heart so is he" tbh I found nothing that contradicts the Christian faith though obviously Christians applying it consciously are more likely to call it faith and trust in God than "law of attraction"
People who believe in it alone rather than as a part of their faith will usually understand it in this way:
Everything holds a vibration, our emotions are different vibrations for instance joy or gratitude is a very high vibration and guilt or depression would be much lower vibrations. Our thoughts and beliefs influence our emotions and in turn that's why they have the power to change our vibration.
The belief in law of attraction is that you attract things which match your vibration, or rather you attract things that you vibrationally match (one person may feel great about money, another may find money a hassle and therefore to them money is a lower vibration) so basically lower vibrations your going to attract more things you don't particularly want or higher you'll attract more things you do. But the vibration is the sum of the subconcious belief (which is perhaps translated as heart in the bible)
There's often a time delay, most people don't experience instant manifestations, thus we usually have a mix of good and bad going on in our lives regardless how positive we are thinking but we can definitely influence our experience through our thoughts
And sometimes it's as simple as changing the way we perceive something alters our experience but the actual facts that happen dont change - for instance,
Fact: my child was rushed to hospital with life threatening illness
My experience using loa consciously: gratitude for paramedics, NHS, nurses, doctors, medicine, my child improving fast as a result of this, my child is well again
My partners experience not using it:
Anger that child is suffering, anger that we've had to go through yet another trial, fear that child will suffer again long after child is recovered
It was the same event experienced in two starkly contrasting ways... But we both attracted the experience we received by our thoughts and beliefs about the situation