Parenthesis is an additional information. The normal progression of a sentence is interrupted by extra information or explanations enclosed in commas, brackets or dashes. The extra information can be a single word, a phrase or even a sentence.

Examples:

We (myself, wife Lorraine and daughters Caroline and Joanna) boarded our boat 'Lynn', a Duchess class vessel barely a year old, at Black Prince Holidays' Chirk boatyard.

The boats have remarkably few controls and we were given a thorough briefing about 'driving' ours–along with advice on mooring, lock operation and safety considerations–by Pauline, who even set off with us for a few minutes to ensure we were confident.

Note:

Depending on the importance attached to it, additional information can be enclosed in brackets, commas or dashes.

Brackets - not important

Connor (Amy's boyfriend) bought the tickets.

Commas - neutral

Connor, Amy's boyfriend, bought the tickets.

Dashes - emphasized

Connor–Amy's boyfriend–bought the tickets.