Could you please explain to me this sentence: "The theory of criticism is related to the theory of poetry." - because I have to write a paragraph about it. Thanks all.
It depends on the context. Generally speaking, a theory of poetry expresses what the formal aspects of poetry can be (rhythm, meter, form or free form, style), what its themes might be (creation, love, human condition) and what purpose poetry serves humanity (imagination, artistic expression, symbolism). A theory of poetic (or literary) criticism expresses which poetry is good, effective, and/or enlightening . . . and why such poetry has these attributes. So, a theory of poetry explains what poetry is and a theory of criticism explains if and why that poetry is effective, according to the criteria of what poetry is. Since both theories contain descriptions of what a poem is and what makes a good poem, the theories will overlap.
There are many different theories of poetry (i.e., Formalist, Romantic, Modernist) and many different theories of criticism (i.e., Feminist, Romantic, Formalist, Postmodern). That being said, you can pair any theory of poetry with any theory of criticism. For example, you could use a Romantic theory of poetry and a Formalist theory of criticism to analyze a Robert Frost poem. Thus, you would look at the Romantic themes (imagination, creativity, nature) and the Formalist qualities (meter, tropes, prosody, etc.).
In the simplest terms, a theory of poetry says what poetry is and a theory of criticism analyzes a poem in some way (each theory takes a different approach). Formalists would analyze the poem's form and structure. Romanticists would analyze the form but focus more on Romantic themes. Inevitably, theories of poetry and criticism overlap because they both touch on questions such as what poetry is, how it functions in society, what makes it effective, etc.
Another example: You might use a Marxist theory of criticism to analyze a 19th century poem described by a Formalist theory of criticism. Since Marxism focuses on ideology and history, you would have to show how ideology of that period in history is structured in the poetic form of the poem. In this case, you would look at structure (form) and historical information. The bottom line is that a theory of poetry (the creation of poetry) and a theory of criticism (study of creation of poetry) inform one another.