Why does Francis Bacon describe his essays as "counsels civil and moral"?
I believe that Francis Bacon gave this title to his collection of essays because that is exactly what he thought they were. He sees these essays as advice (counsels) to young men who were trying to achieve positions of power.
In the essays he addresses a wide range of topics that would have applications for people's moral lives as well as for their civil (meaning more practical and even political) lives.
He discusses such ideas as truth and atheism as well more practical ideas such as how to handle adversity.
Overall, then, these 58 essays are meant to help ambitious young men with issues that are both moral and political/practical. This is why he gave the collection the title he did.
"Counsels" are advices/prescriptions, and Bacon's Essays--published in three volumes--are indeed prescriptive and of advisory nature. These brief but compact pieces were born out of the varied entries in Bacon's Commonplace Books that he used during his programme called "Instauratio Magna." Written in a terse, epigrammatic language, Bacon's Essays were meant for the young diplomats and courtiers who used to hold Bacon in a very high esteem.
Bacon's Essays deal with a wide variety of subjects--"civil" and "moral," that is to say, subjects relating to civil/social/secular domains of life, and subjects relating to moral/ethical/spiritual domains of life. Just a passing look at the table of contents affirms this: Studies, Travel, Love, Death, Empire, Truth and so on.
Bacon is always precise, organised, his Essays being prescriptions, nevertheless full of wisdom and expertise. These pieces, modelled on Montaigne's essays, have proved the effectiveness of Bacon's prose beyond doubt.