1. Words like Metella and Caecilius are nouns.
There are two ways of writing a noun in Latin.
Metella Metellam
Caecilius Caecilium
These two forms of nouns are called the nominative case and the accusative case.
Nominative: Metella Caecilius
Accusative: Metellam Caecilium

2. If Metella does something, we use the nominative case [Metella]
For example: Metella says hello to Grumio.
Metella Grumionem salutat
But if somebody else does something to Metella, we use the accusative case [Metellam]
For example: Grumio says hello to Metella.
Grumio Metellam salutat