What does Psalm 90:15 mean?

On behalf of a people who were often judged for their sin (Numbers 14:33–35), Moses asks God to grant gladness for as long as they have experienced sorrow. Travel through the wilderness was difficult. The region was hot and dry, and at times the people grew hungry and thirsty. Furthermore, violent tribes attacked them. However, God sustained them, and Moses recognized Him as the source of true joy.

Joy is a mark of a genuine follower of Christ. Galatians 5:22 lists joy among the qualities described as the fruit of the Spirit. The apostle Paul suffered numerous trials (2 Corinthians 11:22–29). When he wrote to the Philippians he was a prisoner (Philippians 1:14), but his letter reveals his abundant joy, and he urged his readers to rejoice (Philippians 4:4). Jesus told His followers that they would encounter tribulation in the world, but He also told them to take heart because He had overcome the world (John 16:33). Jesus promised His followers fullness of joy if they would keep His commandments (John 15:10–11).

The apostle James' words in James 1:2 prove it is possible to have joy in the midst of trials. He wrote: "Count it all joy, my brothers, when you meet trials of various kinds."