See on Jam 1:18, see on Act 26:10. Referring to the patriarchs.
Lump
See on Rom 9:21. The whole body of the people. The apparent confusion of metaphor, first-fruit, lump, is resolved by the fact that first-fruit does not apply exclusively to harvest, but is the general term for the first portion of every thing which was offered to God. The reference here is to Num 15:18-21; according to which the Israelites were to set apart a portion of the dough of each baking of bread for a cake for the priests. This was called ἀπαρχή, first-fruits.
Root - branches
The same thought under another figure. The second figure is more comprehensive, since it admits an application to the conversion of the Gentiles. The thought of both figures centres in holy. Both the first-fruits and the root represent the patriarchs (or Abraham singly, compare Rom 11:28). The holiness by call and destination of the nation as represented by its fathers (first-fruits, root) implies their future restoration, the holiness of the lump and branches.