An infinitive phrase is a phrase that starts with "to" followed by a verb. For example, "to shop" is an infinitive phrase.
To help you understand how an infinitive phrase might work in a sentence, let's consider some possible corrections to your original sentence:
* Johnna went to the store to shop. (Here, the infinitive phrase "to shop" acts as the purpose of Johnna's trip.)
* Johnna went to the store to buy some groceries. (Here, the infinitive phrase "to buy some groceries" tells us what Johnna intended to do at the store.)
Let me know if you'd like more examples or have another sentence you'd like to analyze!