A sybil attack targets a network of peer-to-peer nodes by flooding the network with nodes which are all controlled by the same entity. These malicious nodes attempt to form a dominant majority of the network and convince honest nodes of dishonest facts. For example, a vendor on eBay may flood their competition with negative reviews to trick customers.
Bitcoin is not a majority-rule system. Instead, Bitcoin is run by Nakamoto consensus, wherein every node runs exactly the code and ruleset they wish to run. Those who run the code form a network, while those who do not are excluded. Thus, a Sybil attack on Bitcoin is far more difficult to execute because controlling a simple or even supermajority of nodes will not allow an attacker to alter the ruleset used by honest nodes.