Domain names are one of those tech things that non-tech people can only guess are something related to websites. If you know a little more about domain names, you may say that domain names are website addresses. Actually, it is a little of both. Here is a better explanation of what domain names are, what they look like to domain name brokers, and why a broker would want to deal in domain names.
Names and Numbers
Domain names are hidden numbers. If you select a specific name for your website as part of the search terms people type into the search bar, the words/name delivers a website. The website appears as a series of words, followed by a "dot" and a three-letter designation like "gov" or "org." Yet, if you were to take that domain/website name and plop it into a box to copy it and examine it, it will translate into a series of numbers separated by periods. This set of numbers and periods is what is hidden by the words in the search bar or address bar of the website.
What Domain Names Look Like to a Broker
When a domain name broker buys up domain names, he/she is buying sets and strings of numbers with periods (or "dots") in them. They can translate into words, or have words attached to them to transform them into usable addresses. They may have been used previously, and therefore still have words attached to the numbers, or they may never have been used, in which case you can buy a domain name or sets of domain names and attach URLs and words to these sets of numbers after the fact.
Why a Domain Name Broker Invests in Domain Names
Anybody and everybody who creates a website for the purpose of business, blogging, etc. needs a domain name and address if other people are going to find these websites and explore what is on those websites. People have to pay for a website name or domain address, which means that a broker holding domain names for sale can sell what they have and make a tidy profit. If the broker buys domain names by the gross, then the broker can also sell them by the gross to businesses and major corporations who need to expand their websites to include satellite business sites. Additionally, anyone who wants to make some extra money can look into domain name brokerage.