Olive tree and grafted branches
Here’s what it means
God’s covenant blessing is pictured as one cultivated olive tree into which Gentiles are grafted while unbelieving branches are removed.
Why the Bible says it this way
The writer uses a picture to explain one thing by describing it as another.
A helpful detail: Paul warns Gentile believers against pride and preserves distinction between natural and grafted branches. The metaphor should not be used to erase Israel or invent botanical details.