To understand the VXX ETF, you need to understand VIX futures, to understand VIX futures you need to understand VIX, to understand VIX you need to understand options pricing formulas such as the "Black Scholes" formula
Those are your prerequisites. Learn at your own pace.
Short Answer: When you buy VXX you are buying the underlying are front month VIX futures. Limited by the supply of the ETF's NAV (Net Asset Value) units. It is assumed that the ETF manager is actually buying and selling more VIX front month futures to back the underlying ETF.
Assume nobody knows what an options contract should be worth. Therefore formulas have been devised to standardize how to price an options contract. The Black-Scholes formula is widely used, one of the variables in this formula is "Implied Volatility", which basically accounts for the mispricing of options when the other variables (Intrinsic Value, delta, gamma, theta...) don't completely explain how much the option is worth. People are willing to pay more for options when the perception is that they will be more profitable, "implied volatility" tracks these changes in an option's demand, where the rest of the black-scholes formula creates a price for an option that will always be the same. Each stock in the market that also trades standardized options will have implied volatility which can be computed from the price of those options.
The "Volatility Index" (VIX), looks at the implied volatility of MANY STOCK's options contracts. Specifically the "implied volatility of out the money puts on the S&P 500". If you don't know what that quoted part of the sentence means, then you have at least five other individual questions to ask before you re-read this answer and understand the relevance of these followup questions: Why would people buy out-the-money puts on the S&P 500? Why would people pay more for out-the-money puts on the S&P 500 on some days and pay less for them on other days? This is really the key to the whole puzzle.
Anyway, now that we have this data, people wanted to speculate on the future value of the VIX. So VIX futures contracts began trading and with it there came a liquid market. There doesn't need to be anything physical to back a financial product anymore.
A lot of people don't trade futures, retail investors have practically only heard of "the stock market". So one investment bank decided to make a fund that only holds VIX futures that expire within a month. (front month futures). They split that fund up into shares and listed it on the stock market, like alchemy the VXX was formed.
Volatility studies are fascinating, and get way more complex than this now that the VXX ETF also has liquid options contracts trading on it too, and there are leveraged VIX ETF funds that also trade options