Pruned nodes don’t contain the entire transaction history like full nodes. They are useful for people who want to help secure the network but aren’t willing to commit, or don’t have, the computational resource (hard drive space) to act as a full node. Pruned nodes relay blocks at the chain-tip and newly received transactions to their peers. They might save the last couple of blocks, or store nothing but the UTXO. The degree of pruning is variable and chosen by the node operator.