A validator node is a special type of full node that participates in “consensus”. By participating in consensus, validator nodes become responsible for verifying and voting on transactions and maintaining a record of them.
The security of any blockchain or crypto network is underpinned by validator nodes. That is why it is of vital importance to ensure that validator nodes remain both performant and trustworthy.
For details on how validator nodes are selected on Radix, please visit the Radix Overview here.