The EOMONTH function simply identifies the date on which a given month ends e.g. with an input date of the 10th September, it could return the 30th September. To get the last day of the current month, you can simply enter "=EOMONTH(inputdate,0)".
The input date can be a cell reference or a date serial. The "0" is of more interest because it indicates how many months in the future, you wish to calculate the final day. If you enter "-1", you would get the last day of the previous month. This can be useful if you need to calculate maturity dates.
The fact you can convert all dates to the final day of their respective month is a powerful tool should you ever wish to aggregate data. As pivot tables exist purely to aggregate data, this is exactly how "EOMONTH" is used in this training video.