Filters are created with an RPC and later queried through other RPCs.
They allow filtering the blocks/logs or transaction we're interested in querying.

Three types of filter:

• Log filters: eth_newFilter. This has many config options (which blocks we’re interested in, topics, contract addresses). Smart contract logs are translated into ContractEmitted substrate block events. These are easily queryable.
• Block filters: eth_newBlockFilter. This has no params.
• Pending tx filter: eth_newPendingTransactionFilter. This also has no params. We can easily use the import_notification_stream and ready methods on the substrate txpool.