To send Bitcoin, you must have access to the public and private keys associated with the amount of bitcoin you want to send. When we talk of someone "owning" bitcoins, what it actually means is that person has access to a 'key pair' comprised of:
-a public key (an address) to which some amount bitcoin was previously sent
-the corresponding unique private key (a password) which authorizes the bitcoin previously sent to the above public key (address) to be sent elsewhere.
Public keys, also called bitcoin addresses, are randomly generated sequences of letters and numbers that function similarly to an email address or a social-media site username. As the name implies, they are public, so you are safe sharing them with others. In fact, you must give your Bitcoin address to others when you want them to send you bitcoin. The private key is another sequence of letters and numbers, also generated randomly. However, private keys, like passwords to email or other accounts, are to be kept secret. Never share your private key with anyone that you do not 100% trust to not steal from you.