The authors of New Testament books didn't title their works. After all, do you put a title on all your texts? Later generations of Christians ran into trouble though when they realized they needed some names on these to help them tell which book was which. They couldn't use the first lines for these two like they had done for other books, so they went the simple route.
The most commonly used titles for these three books—The Epistle to the Galatians, The Epistle to the Philippians, and The Epistle to Philemon—pretty much say it all. It just means that these books of the Bible are:
• letters…
• that were sent to the churches in Galatia and Philippi, respectively…
• and also to a Christian guy named Philemon.
Sometimes you'll see alternative titles for these three, like "The Letter of St. Paul to the Galatians" or even just "Philippians" and "Philemon." These all mean the same thing. Paul wrote 'em. The early Christians read 'em. Then they wound up in the Bible. Easy peasy lemon squeezy.