Just like an old fashioned grandfather clock, time time on your computer’s clock can slowly drift. You can quickly verify the accuracy of your clock by comparing it to https://time.is/. It’s not unusual for it to be anything from a few seconds to a couple of minutes out. For most purposes this is not a major issue, but there are some applications which are very time sensitive.
NTP (Network Time Protocol) is a tool which will synchronise your computer’s clock with a network of accurate time servers, ensuring that it’s always accurate.
There’s a lot to be said about NTP, but this is a quick guide to getting it up and running on an Ubuntu machine.