Content is the philosophical category which refers to the sum of all aspects, attributes, and processes that a thing, phenomenon, or idea is made from. The Form category refers to the mode of existence and development of things, phenomena, and ideas. Form thus describes the system of relatively stable relationships which exist internally within things, phenomena, and ideas. Content and Form have a dialectical relationship with one another, in which content determines form and form impacts back on content.

Consider a Comic Book. The Form it takes is that of a book, but its content being mostly images accompanied with dialog impacts its form. A comic book of similar page length to a typical book will be considerably easier and faster to read even if all other aspects of the two are the same. Should you try and make a "comic book" without significant illustrations the object is no longer a comic book. If you were to produce a comic book with a similar word count to another book, the size of the comic book would be considerably large compared to the book of similar word count (content impacting form).

Essence and Phenomena (Category Pair)

The Essence category refers to the synthesis of all the internal aspects as well as the obvious and stable relations that define the existence, motion and development of things and ideas. The Phenomena category refers to the external manifestation of those internal aspects and relations in specific conditions. Essence always determines which phenomena appear, but phenomena do not always accurately reflect essence in human perception; in other words, it is possible to misinterpret phenomena, leading to a misunderstanding of essence, or to mistake phenomena for essence.

This one is tricky so I'll quote the book directly:

Essence should not be confused with Form. Form represents the stable internal relations of the component content of a subject, whereas Essence represents the synthesis of all internal aspects as well as all obvious and stable attributes which define the existence, motion, and development of a subject.

Phenomena are simply external manifestations of a subject which occur in specific conditions. The Essence of a subject is not dependent on conditions, whereas in different conditions, the same subject will exhibit different Phenomena. For example, COVID-19 is, essentially, a specific virus strain. That is to say, all of the internal aspects and stable relations that define the existence, motion, and development of COVID-19 are synthesized as a virus which we call COVID-19.

The Phenomena of COVID-19 which we can observe in patients would include symptoms such as fever, coughing, trouble breathing, etc. The Essence of a cloud is water vapor in the atmosphere: that is the synthesis, the coming-together, of all the internal stable relations and aspects which will determine how a cloud exists, moves, and develops over time.

The Phenomena of clouds are all the things we can sense: the appearance of big fluffy white things in the air, shadows on the ground, and, sometimes, rain.

Essence defines Phenomenon: the internal attributes and stable relations will produce the Phenomena which we can observe. A cloud is not essentially defined as a fluffy white thing in the air; that is just the appearance a cloud has to our human senses in certain specific conditions.

The book is called: The Worldview and Philosophical Methodology of Marxism-Leninism: Curriculum of the Basic Principles of Marxism-Leninism Part 1

It's a fantastic resource. You can get the PDF or epub for free. I highly recommend it.