\d is a digit (a character in the range [0-9]), and + means one or more times. Thus, \d+ means match one or more digits.
For example, the string "42" is matched by the pattern \d+.
You can also find explanations for pieces of regular expressions like this using a tool like Regex101 (online, free) or Regex Coach (downloadable for Windows, free) that will let you enter a regular expression and sample text, then indicate what (if anything) matches the regex. They also try to explain, in words, what the regular expression does.