As transactions require the network to commit resources (compute, memory, storage and bandwidth), it is important to ensure that the resources are not abused.
To this end, the resources consumed to process each transaction is measured in a unit called "gas". Therefore, if a transaction handles a simple transfer of native tokens, it will lead to less gas consumed compared to a transaction that executes a complicated smart contract.
Users issuing transactions then have to pay to the network to process their transactions. The cost is proportional to the gas consumed to process the transaction.