I hear people saying, "Bi jah al-Nabi" what does it mean exactly?
The statement literally means, "by the exalted rank of the Prophet ﷺ." The long form is, "we seek more blessings through our belief in the Prophet's great rank." It is rooted in the Prophet ﷺ confirming that anything connected to him brings blessings, which is increase in good. The basis for this is...
In Sahih Muslim, Umm Sulaym was gathering a liquid into a bottle. The Prophet ﷺ asked, what is that? She replied, it is your perspiration; we hope to bring blessings to our children through it. The Prophet ﷺ replied, "You are right to do so."
Al-Bukhari has a similar hadith about the Companions vying for the Prophet's wudu water. The Quran also mentions the Tabut, or the Ark of the Convenant, contained the relics of Musa, Harun, and their families. It brought serenity to the Bani Isra'il, who took it with them whenever they had a battle.
Later Muslims realized, obviously, that we do not have access to the relics of the Prophet ﷺ. However, we do possess many other connections to him ﷺ, such as beliefs in his exalted rank with Allah. So that's what the phrase "bi jah al-Nabi" means: "O Allah we seek Your blessings through our belief in the Prophet's great rank."
Anything connected to his sunna ﷺ brings blessings, as do expressions of belief in him, love of him, and obeying him.
Seeking increase through connection to the Prophet ﷺ is one part of a larger program of acquiring Divine blessings, which include taqwa first and foremost, goodness to parents, giving sadaqa, reciting Quran, making dua, forbidding wrong, and really, all the practices of Islam and general goodness to people.