You find the Arch of Titus at the highest point of the Via Sacra, the main street of the Roman Forum. Emperor Domitian gave the order for its construction in 82 BC to remind of the victories of his deceased brother Titus.
Titus captured Jerusalem in 70 BC. Two reliefs on the inner sides of the arch show scenes of Titus' triumph over Jerusalem. The relief on the inner south side displays how the spoils, including the Menorah, were taken away from the Temple in Jerusalem. The relief on the inner north side shows the celebration of Titus in a chariot - with the personified victory crowing him.
On the side facing the Colosseum, the inscription reads 'Senatus Populusque Romanus divo Tito divi Vespasiani filio Vespasiano Augusto', which means 'The Senate and People of Rome, to the divine Titus Vespasianus Augustus, son of the divine Vespasian'.
Especially during summer, it can get quite hot. Since you can hardly find any shadow it is a good advice to visit the Roman Forum in the morning hours. And bring comfortable shoes because the ground is a bit rough.
I also advise you to book tickets online. Usually, the tickets are sold as combi tickets that also allow access to the Colosseum and Palatine Hill.
Last but not least, you shouldn't miss the view from the top of Palatine Hill. From there, you can nicely overlook the whole Roman Forum.