Learn Croatian: Ona može plesati.

Because it follows the verb može (can). Just like in English, when you use a word like can, the verb that comes right after it stays in its basic dictionary form (called the infinitive). That is why we use plesati (to dance) instead of a conjugated form.

Is it mandatory to include the word ona in this sentence?

No, you could simply say Može plesati. Croatian verb forms already give a strong clue about who is doing the action, so subject pronouns are often dropped. Using ona here just adds a little emphasis, making it perfectly clear that she can dance, as opposed to he or it.

What is the base dictionary form of može?

The base form (infinitive) is moći, which means to be able to. Može is the specific present tense form used for he, she, or it. It is an irregular verb, so you will notice that forms like može look quite different from the dictionary form!

How do I recognize that plesati is in its basic dictionary form?

You can tell because it ends in -ti. The vast majority of Croatian verbs end in -ti in their dictionary form. A smaller group of verbs ends in -ći, just like the verb moći itself.