A public key is a lengthy sequence of characters that is used in cryptocurrency and other digital systems to receive assets or to authenticate digital signatures. It is a part of the key pair that is generated through cryptography: a public key and a private key. The public key is intended for wide distribution, whereas the private key is the one that must be kept to oneself. They enable secure communication and transactions on a blockchain.
In the case that someone wishes to transfer cryptocurrency to you, the sender will make use of your public key (or a wallet address derived from it) to send the funds to the correct location. The key may be seen by everyone, but it is still not capable of being used to get or transfer your assets—that is only done with the private key. This is how the secure system works: by letting people share public information without the risk of losing ownership.
Moreover, public keys are instrumental in the process of validating digital signatures. If a user signs a transaction with his private key, anybody can use the public key to verify that the signature is legitimate and that no alterations have been done to the transaction.
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To put it simply, a public key is like an email address which you can provide to others. The sender can send you data or cryptocurrency, but only you can access it with your private key.