1After this Paul left Athens and went to Corinth. 2And he found a Jew named Aquila, a native of Pontus, recently come from Italy with his wife Priscilla, because Claudius had commanded all the Jews to leave Rome. And he went to see them, 3and because he was of the same trade he stayed with them and worked, for they were tentmakers by trade. 4And he reasoned in the synagogue every Sabbath, and tried to persuade Jews and Greeks. (Acts 18:1-4 ESV)

A commentary can provide a bit of background as to what this profession was, why Paul was trained in it and why he performed the work:

Jamieson-Fausset-Brown Bible Commentary

3. tentmakers-manufacturers, probably, of those hair-cloth tents supplied by the goats of the apostle’s native province, and hence, as sold in the markets of the Levant, called cilicium. Every Jewish youth, whatever the pecuniary circumstances of his parents, was taught some trade (see on [2044]Lu 2:42), and Paul made it a point of conscience to work at that which he had probably been bred to, partly that he might not be burdensome to the churches, and partly that his motives as a minister of Christ might not be liable to misconstruction. To both these he makes frequent reference in his Epistles.

Notice what the end of this section of commentary points out.  Throughout Paul’s epistles, he makes reference to the work that he does to support his ministry, so as not to be a burden on the churches or to be accused of doing the ministry for money.

As we can see in the Book of Acts, and throughout his epistles, Paul’s calling was to preach the Gospel of Jesus Christ, first to the Jews and then to the Gentiles.  Yet, he had another profession that he performed to support himself and his ministry – that as a Tentmaker by Trade.