“Stoicism” by John Sellars is an excellent introduction to the Stoic Philosophy.
In the chapter regarding Stoic Logic, Assent is described in the following way:
…the impression of a man sitting under a tree is presented to the mind as a proposition such as “there is a man sitting under the tree”. The mind then accepts or rejects this proposition. This is an act of assent.
What do we see in the world around us and what do we acknowledge and accept?