The answer is not unique to bitcoin. It would be the same if you're dealing with (non-crypto) foreign currency, stock, a stock derivative or commodity or commodity futures.

When you buy something like the above, you are giving up your "real money" (fiat currency) to take possession of said commodity/stock/bitcoin/etc (let's call these assets in general). You bought an asset at a particular market value, which can fluctuate over time. If it rises, it's worth more of the fiat currency. If it falls, it's worth less of the fiat currency.

While you're still holding on to said asset, what you're experiencing are called unrealised gains and unrealised losses. The valuation chart fluctuates, but you're not seeing your purse of fiat currency changing in any way after the initial outlay.

You won't actually feel the "pain" of a fall in value until you decide to sell the asset. At this point you will experience a realised loss. You will get back less of the fiat currency (real money) than you put in in the first place. Conversely, if the asset has risen in value, you'll get back more "real money" than you put in and you've made a realised gain.

(I simplified the analysis to omit things like trading overheads - brokerage fees and commissions, etc. that always apply (those things always eat at your gains and losses pretty much equally). And taxes are a complex and murky thing I won't touch. And all this assumes you paid "real money" you actually had in full for the asset. If you borrowed money to buy the asset, that's called trading on margin and it can be much, much riskier - you're losing money in interest all the time and your losses can be more than the amount you borrowed to begin with. But ignore all these complications and focus just on the paragraphs above to give you a head start in understanding. And please learn more (and try trading simulations) before you trade real money for any asset).