A second is a measure of time. They use an atomic clock now to measure time with accuracy. down to 13 digits as I understand. They can divide a second up into over a million points of pin point accuracy.
While they can break down the second to 13 digits. The second is also a breakdown of a larger number, The minute, having sixty of those seconds in it and up again to the hour which will have 3600 seconds in it.
A second can be seen with each tick of the clock. A second hand on the clock is useful to see this. My age is showing here lol. The seconds can be seen adding up on digital clocks today and resetting when they reach 59 back to a 0. There are many ways to measure time.
You can see different ways to measure time here, https://www.timeanddate.com/clocks/free.html
There are also some free clocks for people to take advantage of. We are currently looking into taking a clock into our discord server to avoid the need for alternate time zones.
A second is the smallest common standard for measuring time on a daily basis.
Please note, When the wife says just a second, this can mean anything up to two hours.