How do you use units and convert them to understand and solve problems

Units are used as a standard for measurement of the same quantity. It is a definite magnitude of the quantity that is defined by a convection.

Without them we could not be able to measure things.

All physical quantities have their units, For example: length is measured in meters, weight is measured in kilograms, power is measured in watts, ...

With units we can understand "physical" problems and have an exact answer of the problem.