In plain English
This is just the dollar price of one bitcoin over time. When the line goes up, bitcoin got more expensive in dollars.
The vertical axis is usually logarithmic. That sounds fancy, but it only means equal height = equal percent change. A jump from $1,000 to $10,000 looks as tall as $10,000 to $100,000, because both are a 10× move. Use it as the baseline tape, not as a buy/sell signal.