If a person is sui juris then they are of age (=no longer subject to parental authority) and therefore able to manage their affairs. A person who is sui juris is legally competent and has full civil rights.
Sui juris is considered to be legalese (=the formal and technical language of the law that is often difficult to understand, especially for non-lawyers). Legalese often confuses the reader, so good legal writers use plain English whenever possible. As is the case with many Latin terms, sui juris should be avoided whenever there is a suitable alternative. Here, that alternative is legally competent.
Sui juris is often used in the context of trusts. For example, a sui juris beneficiary of a trust is a beneficiary that is not under a legal disability and is deemed to be mature enough to handle their own affairs (=business or concerns), and may include the following: a court-appointed guardian; an agent for an incapacitated beneficiary; or the parents of a minor beneficiary.
Sui juris is sometimes confused with sui generis, which means unique, or in a class of its own.