Dual mining describes the way of mining two types of crypto simultaneously, or alternately mining within a certain period of time by utilizing the same amount of hashrate.
Simultaneous dual mining is commonly seen for GPU mining. Miners configure the mining pool server URLs of two coin types in the mining software. When the mining starts, part of your hashrate will mine Coin A, meanwhile the rest of the hashrate mines Coin B. Miners have to allocate hashrate to determine which coin will be mined more. An example of this type of dual mining would be mining an Ethash coin and Kaspas together. Read tutorial.
Dual mining alternately, is to mine the main coin type that you selected and then switch to another coin type for a fixed amount of time, then switch back. In the given time span, your hashrate can only mine one type of coin. An example of this type of dual mining would be ETC+ZIL dual mining. Read tutorial.