This is the base of a monument that the inhabitants of Lepcis Magna dedicated to Gaius Gavius Macer, a Roman senator from Verona in Cisalpina Gaul who is known to have been quaestor in 19 CE. There must have been a statue on top of it: almost certainly an equestrian statue, because the remains are too large for a normal one.
In other words, Gaius Gavius Macer received high laudes from Lepcis' municipal administration. He must have played a role in the war against Tacfarinas, a local leader who had organized several Numidian and Mauretanian tribes in an anti-Roman coalition.
In the inscription, Gavius is mentioned as legatus pro praetore, which was the name of the right-hand man of the governor of Africa, who had the rank of legatus pro consule. Probably, Gavius was the deputy of Gaius Vibius Marsus (who dedicated this inscription to Augusta Salutaris).
Gaio GAVIO MACRO
LEGato PRO PRaetore III
Which can be translated as:
To Gaius Gavius Macer
for the third time legatus pro praetore,
on behalf of all the Lepcitanians.