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. Critically, the method for determining the valid state of the blockchain is objective, and does not rely on the opinions of other nodes. 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.