Definition
Encryption hides content, not patterns. An observer who can watch both ends, for example your ISP and the website's network, may link your encrypted stream to the request that comes out of a VPN or Tor exit by comparing when packets are sent and how big they are.
How it works
Attackers compare packet timings, bursts and sizes across many flows and look for statistical matches. The Tor Project lists this as a known limit of low-latency anonymity networks. Countermeasures include padding, adding cover traffic and routing through several jurisdictions.
Why it matters
It is the realistic ceiling on what any VPN or Tor can promise against a very powerful adversary. Some providers now add traffic padding or random delays; for most people, the threat model is advertisers and ISPs, not global observers.