The imposing arch located on the Via Sacra in the Roman Forum commemorates Emperor Titus and his military successes during the Siege of Jerusalem. It was installed in 82 AD by order of Emperor Domitian, who was Titus’ younger brother. It is considered to be one of the world's oldest triumphal arches.
There are many arches scattered all over the world modelled on this one. The most famous ones are e.g. the Arc de Triomphe in Paris or the Washington Square Arch in New York.
Since it is part of the Roman Forum, you can get close to it only if you visit the Roman Forum itself (and pay the entrance fees). You can also see it from all higher points around (e.g. from the Colosseum or from the Palatine Hill).