Percentages are used for all sorts of things, such as rates of VAT and other taxes, the chance of something happening, for example the probability of rain today, and demographics, e.g. in 2018 18.3% of the UK population was aged 65 or over.
Although most of us are familiar with, and comfortable with percentages it's worth noting that there are other ways of saying the same thing. Fractions, and ratios tell exactly the same message. A percentage is a proportion, or fraction, expressed in hundredth. So 1% is equivalent to 1 in 100, or 1/100th.
10% is equivalent to 1/10 or 1:10 50% is equivalent to 1/2 or 1:2
Depending on your interests or work you might be more familiar with one of these other styles, so just remember they are equivalent and can be converted from one to the other with some arithmetic.
In some areas of science and engineering it's common to see figures such as 0.01% or 99.99%
When a quantity is expressed as a percentage we understand that this value will apply in the same proportion for all numbers. For example if we are told that 1.13% of people in England who were tested were found to have Covid-19 then we can say that for every ten thousand people 113 will have Covid-19, because we can know that ten thousand is 100 x 100, so if multiply 1.13 by 100 we can calculate the number of infected people. Of course this is only an estimate, but such estimates can often be quite accurate and very useful, after all it would not be practical to test everyone, every week. If you know the number of people in your town, you can calculate the likely number of infected people.
See https://www.ons.gov.uk/peoplepopulationandcommunity/healthandsocialcare/conditionsanddiseases
Although we can use percentages to estimate the number of people in our town who are infected right now if we have test results, we cannot use them to estimate the number of infected people in coming days and weeks. For this we will need to understand how to estimate exponential growth.