This is a comparison to the young plants that have very shallow roots. This metaphor means that the people were first excited when they received the word, but they were not strongly devoted to it. Alternate translation: "they are like young plants that have no roots" (See: [[rc://en/ta/man/translate/figs-metaphor]])
It may be helpful to explain that tribulation comes because people believe God's message. Alternate translation: "tribulation or persecution comes because they believe God's message" (See: [[rc://en/ta/man/translate/figs-explicit]])