regexps are a shorter word for regular expressions and they are used to find patterns in data so you can check for valid input or extract portions of data that will be different everytime but fall in the same pattern.
for example you can use a regular expression to make sure an email address someone types is in the valid format, someone@somewhere.tld, or check for a valid mm/dd/yyyy date. the options are really endless.
see this for more info and examples.
