Only a high priest could offer sacrifices so that God could forgive sins, so Jesus had to be a high priest. The law of Moses commanded that the high priest be from the tribe of Levi, but Jesus was from the tribe of Judah. God made him a priest like the priest Melchizedek, who lived at the time of Abraham, before there was a tribe of Levi.
The writer speaks of Christians who are only able to understand simple things about Jesus as if they were babies, who drink only milk and cannot eat solid food. (See: [[rc://en/ta/man/jit/figs-metaphor]])