SPV clients lack the ability of full nodes to detect whether or not a chain provided to them by another source complies with the rules of the Bitcoin protocol.
SPV clients can connect to multiple full nodes in the hope that at least one of the nodes is honest and will provide them with the best valid chain, however situations may arise where the non-compliant chain contains more proof of work than the compliant chain. In this situation, there is no way for the honest full node to signal to an SPV client that it should disregard the chain with more proof of work.
Fraud proofs are a technique which provide honest full nodes the capability to conclusively demonstrate that chain is invalid regardless of the amount of proof of work backing the invalid chain.
If Bitcoin nodes implement the ability to create, propagate, and verify fraud proofs, the security of SPV clients will be improved.