A more thorough explanation:

An alternative promise is a type of contractual promise where the promisor agrees to do one of two or more things, and any one of those things counts as consideration. For example, if a person promises to either mow their neighbor's lawn or clean their gutters, and the neighbor agrees to pay them for either task, that is an alternative promise.

According to the Restatement (Second) of Contracts, an alternative promise can be made because each of the alternative performances is desired by the promisee, or the promisor may reserve an alternative that they deem advantageous. In either case, the promise is considered consideration if it cannot be kept without some action or forbearance that would be consideration if it were separately bargained for.

For instance, if a person promises to either paint their friend's house or fix their car, and the friend agrees to pay them for either task, that is an alternative promise. If the promisor chooses to fix the car, they cannot do so without incurring expenses for parts and labor, which would be consideration if it were separately bargained for.