A trading pair refers to the value relationship between 2 assets, a BASE and a QUOTE asset, which determines the actual EXCHANGE RATE against each other.
The PRICE/VALUE represents how many units of the quote currency are equivalent to 1 unit of the BASE asset.
In crypto exchanges, the most common type of pairs use USDT or $USDC as a quote asset.
The price fluctuations reflect the change in relative value.
For example, in the case of BTC/USDT (BASE/QUOTE), a price of 100K means you need as much USDT to acquire 1 BTC.
The concept is simple, but it can be confusing when exposed to more arcane pairs, such as ETH/BTC, where BTC is used as quote asset, because people think in terms of fiat.
Trading CROSS PAIRS such as these is complex, when the 2 assets are volatile against stable coins as the notion of relative value chases a fast moving target.
There are very exotic pairs out there. Have you tried them?
Let me know which is the rarest you have found so far!