Hi, James —
It's more or less true.
We believe that for many people, probably most, a period of purification from attachment to sin and worldly things is necessary before one can enter Heaven (See 1 Corinthians 3:10-15).
We call this state of cleansing Purgatory. Having a Mass offered for the individual can help expedite that process by applying the merits of Christ on the Cross to them. It's also a natural expression of love for the individual to pray for them (which is what we are doing when we have a Mass said for them).
It's not required but it is very helpful.