We always measure, and measurement is our daily life. How do you measure how long something is? You need to make the standards of length to say a body has some length. And the standard you use to give a physical quantity a measurable value is unit. Now you can say the body has a certain length using your length standard.

We measure a physical quantity by comparing with the unit of the physical quantity. A unit is the standard form of a physical quantity that allows you to measure it. Anyone is free to make any standards for measurement, so there must be a universal system of measurement accepted worldwide and anywhere in the universe called metric system or SI units.

For example, you made a stick of a certain length and measured other lengths with that stick and found the length of your bed to be five times that stick. So here the length of the stick you made recently is your temporary length standard (you used it to measure other lengths).

When we make a standard to define a physical quantity, that becomes a unit. You can give your own standard to any physical quantity, for example if you made the period of a particular pendulum to be your time standard and calculated other time periods like day, month or year, that time period will be based on your own time standard.

Anyone is free to make any standards for units, so it's very important to define universal standards (the same standards in the entire world) for units called SI units (*system international* units). You may not be interested in the history of SI units but a little bit of skim on it can provide you a way how they approached to their current operational definition.