During event set up a key component is defining your ticket's innovatory and prices. When you add a ticket type, you will define it's name, date's of sale, quantity, & it's available prices. These are what are referred to as price levels.
For example, let's say your ticket type is a "Friday Ticket." You have a capacity of total tickets that can be sold, but you might have different prices within that, eg: Adult & Child. So within that Friday ticket, there is an adult ticket for $15 and a child ticket for $10.
Tip: A price level doesn't have to be available to the public. Sometimes you might have a price that is hidden. For example, if you have a student price but students need to show their id in order to buy the ticket. In this case, just select to not have it available online.