Romans 11:16-18 — For if the firstfruit be holy, the lump is also holy: and if the root be holy, so are the branches.
And if some of the branches be broken off, and thou, being a wild olive tree, wert grafted in among them, and with them partakest of the root and fatness of the olive tree;
Boast not against the branches. But if thou boast, thou bearest not the root, but the root thee.
The quote in verse 16 is from Numbers 15:21 — Of the first of your dough ye shall give unto the LORD an heave offering in your generations. Israel was told to offer the first of their dough to God as an heave offering.
firstfruit and root — Abraham, since he was set apart (holy) to God, so is Israel, which came from him
branches broken off — unbelieving Jews
wild olive — Gentiles
grafted in — admitted into God's favor, which Israel has forfeited for a time.
partakest — Gentiles became Israel's spiritual descendents
boast not — Gentiles shouldn't consider themselves better than Jews or hold them in contempt — their salvation comes through Abraham.