It's used to introduce something that is present, either literally or figuratively.
Here are some examples:
* Voici mon livre. (Here is my book.)
* Voici les clés. (Here are the keys.)
* Voici ce que j'ai trouvé. (Here's what I found.)
* Voici le problème. (Here's the problem.)
It's important to note that "voici" is always followed by a noun or pronoun.