I view your question in two ways one being from the point of you not knowing what bitcoin ATH mean and the other being you wanting a clarification on how to know it before hand. For the first part, bitcoin ATH simply means the point bitcoins records a high it has not recorded before. For now the ATH of bitcoin is $73,738. On the aspect of knowing the ATH before it happens, I don't think there is anyone that can know that with accuracy. The best people do is to predict using past data and gauging the market momentum combined with the news and development within the bitcoin ecosystem. Even with all these tools, it is still impossible to know what the ATH will be before it happens.
To be fair, a situation where we end up with a "prediction" regarding all time high, is literally the speculation that we all make here. This is the forums speculation part, and when you say it will go up or down, you make a speculation, when you say it will be 100k in 2 years, you make a speculation, when anyone says "I believe we will break ATH price in 6 months" they are making a speculation.
You do not have to know it, nobody knows it, but just because we do not "know" it, does that mean we can't trade? Trading is literally turning the speculation into execution and that means we are going to end up with a greater return and that's the most important part. I think this is why we need to realize that things aren't always that easy and just have to go with the flow even if we don't know.