Some translations indent each line of poetry to make it easier to read. The ULB does this with the poetry in 25:23.
Abraham married another wife. This was sinful of him. Many other Hebrew leaders came to marry multiple wives. This is known as "polygamy." It has never been an acceptable practice. (See: rc://en/tw/dict/bible/kt/sin)
In the ancient Near East, inheritance was passed on through the eldest son. Even though Esau and Jacob were twins, it is very important that Esau was born first. It was his birthright to receive his inheritance, but he foolishly gave it up. (See: rc://en/tw/dict/bible/kt/inherit, rc://en/tw/dict/bible/kt/birthright and rc://en/tw/dict/bible/kt/foolish)
God begins to fulfill the promise he gave to Abraham about making his descendants numerous. Many of these children through his concubines would become large people groups. This may not be easy to understand without the remainder of the Old Testament and the history it records. (See: rc://en/ta/man/translate/figs-explicit)