A bitcoin transaction is valid if it conforms to all of the bitcoin protocol rules. Likewise, a block is valid if it conforms to all of the bitcoin protocol rules (including: all of the transactions it contains are valid).
‘Validation’ of a block or transaction happens at the level of each individual node (miners and regular nodes alike). Validating a transaction or block is not that computationally expensive, in fact the protocol is designed so that fairly inexpensive hardware is capable of being a node (ie, your average computer can validate hundreds, or thousands of transactions per second).
Upon hearing about a new transaction or block, each node will quickly validate that it conforms to the network’s rules. If it is valid, that node may relay the transaction/block to its peers. If it is not valid, the node will reject that transaction/block, and not add it to the node’s mempool/blockchain.
the average time it takes to validate a block by the network is 10 minutes.
This is not quite technically correct, I would rewrite it to say:
the average time it takes for miners to find a new block on the network is ten minutes. Each block ‘confirms’ all transactions it contains. As the new block is transmitted across the network, each node will check it (and all of its transactions) to ensure they are valid according to the protocol’s rules.