John Gill Commentary 2 Chronicles 11:22

"And Rehoboam appointed Abijah the son of Maacah to be chief, [even] the prince among his brethren; for [he was minded] to make him king." — 2 Chronicles 11:22 (ASV)

And Rehoboam made Abijah the son of Maacah the chief
The chief of all his sons, head over them, being the son of his most beloved wife, and her firstborn however, and perhaps might be of a greater capacity than the rest of his children:

to be ruler among his brethren ;
or over them; gave him greater authority, and entrusted him with more power, set him above them, and treated him as heir apparent to the crown: for he thought

to make him king ;
either to take him into partnership in the throne with him in his lifetime, or to appoint and declare him to be his successor.