Pay-to-Public-Key (P2PK) is a type of ScriptPubKey which locks bitcoin to a public key. This means that the bitcoin can only be spent by the owner of the private key corresponding to the public key provided in the script. A public key in this case can be thought of as a P.O. Box, where anyone can send mail (bitcoin) using the address (public key) of the P.O. box, but only the owner of the key can access the mail thereafter. A P2PK transaction is a transaction whose inputs used a P2PK ScriptPubKey.
For example, if Alice sends 1 BTC to Bob in a P2PK transaction, she includes Bob’s public key in the transaction. When Bob wants to spend the bitcoin he received from Alice, he must sign his transaction with the private key corresponding to the public key Alice included in her transaction.
Technically, P2PK is simply the name of a particular script with the aforementioned requirements. Every Bitcoin output has a script that determines how the output can be spent, and P2PK one such script. P2PK scripts were deprecated in favor of P2PKH scripts for superior security and convenience.