P2WPKH-in-P2SH together with P2WSH-in-P2SH represents type of transaction which allows non-SegWit wallets to generate a SegWit transaction. It uses a P2SH transaction as the cornerstone and includes P2WPKH inside it. If a SegWit-aware node sees a P2SH which contains P2WPKH it interprets it in the native way.
ScriptPubKey: OP_HASH160 <20-byte-redeemScriptHash> OP_EQUAL ScriptSig: <0 <20-byte-PublicKeyHash>> Witness: <Signature> <PublicKey>
RedeemScript: 0 <20-byte-PublicKeyHash>
To understand how transactions work on basic technical level, check blog post Types of Bitcoin transactions: Part I, Part 2. More detailed description can be found in Bitcoin Wiki or in Bitcoin.org Developer guide.