Learn Portuguese (Portugal): Se não compreenderes o esquema, posso explicar. - If you don't understand the diagram, I can explain it.

This is the Inanimate Null Object syntax you are currently learning! In European Portuguese, when the direct object is an inanimate thing that was just mentioned (like o esquema), you can completely leave out the pronoun. In English you must say "explain it," but in Portuguese, simply saying posso explicar is perfectly natural.

Would it be wrong to say posso explicá-lo instead?

Not at all! Saying posso explicá-lo is grammatically correct and means the exact same thing. However, dropping the pronoun for inanimate objects is extremely common in everyday spoken European Portuguese, so mastering this will make your speech sound much more natural.

Why do we use compreenderes instead of the regular present tense compreendes?

The word se (if) triggers the future subjunctive tense when setting up a hypothetical future situation. While English uses the present tense "If you don't understand," Portuguese requires the future subjunctive compreenderes because the condition is an open possibility.

Does this pronoun-dropping rule apply to people as well as objects?

No, this is why it is called the inanimate null object. If the object you are referring back to is a person or an animal, you must use the pronoun. For example, if you wanted to say "If you don't see the doctor, I can call him," you would have to use the object pronoun for "him" attached to the verb, never just the verb alone.