What did Paul teach Apollos and Priscilla about the gospel of grace?
Paul taught that the gospel of grace is that everything man needed was accomplished on the cross, along with the power of Christ's resurrection. He unloaded this on Apollos and Priscilla for weeks; Apollos had known none of it. Priscilla and Aquila then passed it on, having received it from working with Paul.
Les teaches that Apollos and Priscilla were sitting with Paul for weeks, and Paul unloaded on them the tremendous gospel of grace—how everything man needed was accomplished on that cross and the power of his resurrection. Apollos knew none of that. Les uses this to encourage people to kindly show their pastor from Scripture what he may be missing; if he is an Apollos, he should receive it humbly. Apollos was highly educated, polished, and eloquent in the Scriptures, yet from these two common tentmakers he realized there was much he did not know. As he finished ministry at Ephesus, he took to heart what he learned from this Jewish couple. When he was disposed to pass into Achaia, brethren wrote exhorting believers to receive him, and he helped much those who now believed through grace. He got it from Priscilla and Aquila, who got it from working with Paul.