A node runs the Bitcoin Core software (or another reference implementation) that allows users to interact with and participate in network consensus by maintaining a copy of the Bitcoin blockchain and validating transactions and blocks.
Note while all miners are nodes, not all nodes mine bitcoin. Importantly, running your own node lets you independently enforce Bitcoin's protocol rules by validating blocks and transactions yourself, rather than trusting a third party. This means it can reject blocks or transactions that do not comply, enforcing Bitcoin's protocol rules independently of any central authority.
An example of node runners attempting to enact an update or change to the Bitcoin network was during the 2017 'Blocksize Wars,' where node runners supported a User-Activated Soft Fork (UASF). Users enforced SegWit's activation independently of miners by running node software that rejected blocks failing to signal support, pressuring the network toward accepting the protocol change.
In addition to participating in the governance of the Bitcoin network, running your own Bitcoin node provides a user with increased security as you are not dependent on a third party as well as privacy, since your wallet doesn't need to query a third-party server that can see and log your addresses.
Users can download the open-source Bitcoin Core software on its official GitHub repository.