Well usually parsing is a process when you get different kinds of unordered information from some external sources. It could be an output from an external database, website pages or usually just a file. Data can be represented in a mixed way, but you have to get this data and moreover you have to add it to your specific structure (classes, variables and etc.).
Let's talk specifically about this. We have a huge file of some trading data. We don't need the most part of the information in it just some specific numbers but the information is mixed. We see some patterns of it. That's where regular expressions work out perfectly. You can use it in many programming languages and they are the most efficient way to do text parsing.