Learn Italian: Il pulsante è lì, toccalo subito! - The button is there, touch it immediately!

It is a combination of tocca (the informal command for touch) and the pronoun lo (it). When giving a positive command to someone informally, object pronouns attach directly to the end of the verb to form a single word.

Why does toccalo use lo instead of la?

The pronoun lo stands in for il pulsante (the button). Because pulsante is a masculine noun, we have to use the masculine singular pronoun lo. If we were talking about a feminine noun like la mela (the apple), we would say toccala.

I thought the tu form of -are verbs ends in -i. Why is it tocca instead of tocchi?

In the normal present tense, you are exactly right: you touch is tu tocchi. However, affirmative commands (the imperative) have a special rule. For -are verbs like toccare, the informal command form ends in -a.

Why is there an accent mark on lì?

The accent distinguishes the location word lì (there) from the masculine plural pronoun li (them). They are pronounced exactly the same, so the accent mark is just used in writing to tell them apart.