Small proteins, basically.
Link a few amino acids together and you have a peptide, usually somewhere between 2 and 50 of them. Keep going into the hundreds and the chain folds up into a protein.
Three things worth knowing.
Your body already makes them.
Many of the signals your body sends are peptides. Insulin, which keeps your blood sugar in range, is one: 51 amino acids long.
Most can’t be swallowed.
Your stomach breaks peptides down the same way it breaks down the protein in a meal, so most would never reach your bloodstream. That is why they are usually injected just under the skin.
Not every one is right for you.
Some peptides are approved medicines. What makes sense for you depends on your health, your goals and anything you already take, and that is a question for a provider, not the internet.