Offspring is a kicker-like ability you choose when casting a creature spell. You pay an additional cost (usually a set amount of mana listed after "Offspring"), and if you do, when that creature enters, you also create a 1/1 token copy of it. The token has the same characteristics—including abilities—but with its power and toughness set to 1/1.
The common mistake: the token copies the printed creature, not any counters, Auras, or other modifications, and crucially it's locked to 1/1 base stats. People also forget the token is a copy, so it carries triggered/activated abilities and even other instances of Offspring text—though you can't re-pay Offspring on a token already on the battlefield.
In Commander, Offspring shines with token doublers, "enters" triggers, and sacrifice synergies, letting one card generate two bodies and double up on value engines.