A game represents one battle. The battle can be historical, historically possible, or an impossible matchup between armies separated by time or space. It takes about 2 to 5 hours to play and is typically played by 2 to 6 people. A bit less if you can buy it in or order it from England. After that, you can begin with cardboard markers that you make to fit the element sizes described in the rules and if you like the game, buy and paint your first army of soldiers to replace the cardboard pieces.
Some people paint and sell finished armies.
There are people playing the game in most English speaking countries and in several other countries like France, Spain, Italy, Sweden, Finland, Germany, etc. The company that publishes DBM is named Wargames Research Group and is in England. They have been the dominant publisher of miniatures rules for ancient era wargames since the 70s. There are popular competing sets of rules from Arty Conliffe - Tactica and Armati, known for being easier to read than WRG ancients rules, but while they have some ardent adherents, more people still prefer WRG 7th or DBM, and a variety of other rules sets from other publishers for playing battles in the ancient era and middle ages. The nice thing about gaming with miniatures is once you have painted some troops, trying a new rules set is a relatively small investment.