The verb "to be," to be exact. Present tense.
</i>sum, esse, fui, futuris</i> - To Be
1st singular - sum ("I am")
2nd singular - es ("you are")
3rd singular - est ("he/she/it is")
1st plural - sumus ("we are")
2nd plural - estis ("you are")
3rd plural - sunt ("they are")
You may notice some similarities to the French or Spanish. Good observation. Please make special note that the word "est" is pronounced with the T sound. Few things are more annoying than someone who pronounces "est" like the French do. Well, if you're trying for Latin. If you're trying for French, obviously it's okay.
This is an irregular conjugation, but it forms the base for so many OTHER irregular verbs that it is almost a whole conjugation family on its own. So please remember it. It's important.
esse, non esse?