In legal terms, a voidable contract refers to a type of contract which can be voided by one of the parties at any given time without repercussions for them. This should be differentiated from a void contract which is a different case and refers to contracts which aren't legal by default, and thus cannot be enforced through any legal channels. Void contracts can occur in various situations, and are mostly related to performing a contract with a minor.
Minors are a special case in law, as they cannot be held legally responsible for not abiding to the terms of contracts they've signed, and they have the legal right to void contracts they've performed with adults. This makes it a very tricky business for companies providing services aimed at children, as such contracts can only put the company under legal obligations, but not their clients. The most commonly used solution to this problem is to simply require the signature of a consenting adult when performing contracts with children - that way, there's a legal party which can be prosecuted in case a need arises.
Apart from cases where minors are involved, voidable contracts aren't very common in law, and are usually related to the criminal background of individuals, in cases where it may prevent them from engaging in certain types of activities and/or business. However, in those cases, these individuals can still usually be prosecuted through alternative channels related to their activities.