Rules are a means of extending or manipulating the "base" words in a wordlist in ways that are common habits for users. Such manipulation can include toggling character cases (e.g. a to A), character replacement (e.g. a to @) and prepending/appending characters (e.g. password to password!). This allows our wordlists to be overall smaller in size (because we don't have to store every permutation), but with the drawback of a slightly slower cracking time.
hashcat.exe -a 0 -m 1000 ntlm.txt rockyou.txt -r rules\add-year.rule
acbfc03df96e93cf7294a01a6abbda33:Summer2020
Where:
-r rules\add-year.rule is our custom rule file
The rockyou list does not contain Summer2020, but it does contain the base word, Summer.
The hashcat wiki contains all the information we need to write a custom rule that will append the year 2020 to each word in rockyou. We can see that to append a character, we use $X - therefore to append "2020", we just need $2$0$2$0.
PS C:\> cat hashcat\rules\add-year.rule
$2$0$2$0
Hashcat also ships with lots of rule files in the rules directory that you can use.