nElder “Elder” means an older person. It can also mean a special leader in the church (as in Titus 1:5).
nlady This might mean a woman, or in this letter it might mean a church. If it is a church, then “her children” would be the people of the church.
ntruth The truth or “Good News” about Jesus Christ that joins all believers together.

In other versions

The elder to the elect lady and her children, whom I love in truth, and not only I, but also all who know the truth,

The elder,

To the lady chosen by God and to her children, whom I love in the truth—and not I only, but also all who know the truth—

The elder:

To the elect lady and her children, whom I love in the truth—and not only I, but also all who know the truth—

This letter is from John, the elder.

I am writing to the chosen lady and to her children, whom I love in the truth—as does everyone else who knows the truth—

The elder, to the elect lady and her children, whom I love in truth—and not I alone, but also all those who know the truth—

The elder unto the elect lady and her children, whom I love in the truth; and not I only, but also all they that have known the truth;

Public Domain