Strap-on play involves using a harness and dildo combination that allows one partner to penetrate another. This enables penetrative sex regardless of anatomy and opens various possibilities.
Pegging specifically refers to a woman using a strap-on on a male partner. The practice has gained mainstream visibility and is enjoyed by people of all orientations and gender identities.
Harness styles include underwear-type, jockstrap-style, and thigh harnesses. Dildos vary in size, shape, material, and features. Finding compatible harness/dildo combinations and proper fit takes some experimentation.
For receiving partners, standard safe anal or vaginal play guidelines apply - adequate lubrication, gradual sizing, communication, and cleanliness.