Trading pairs are the available exchanges on an exchange. For example, some tokens may only be traded for a handful of specific other tokens. To sell for cash, you often must trade your token for another; perhaps trade that one for Bitcoin and sell Bitcoin for cash. Trading pairs also impact valuation. If your trades are for Ripple and later Ripple goes down in value, it will appear that your coin also goes down in value unless new trades have happened that show your exchange rate with Ripple changing. This fact is important because it reduces the confidence you should have in the crypto values listed at popular exchange sites. When the values can change without new trades providing information, it calls into question the values listed.