Mega'and(select*from(select Sleep(2))a/**/union/**/select 1)=' -

: This is used to combine the results of the original query with a new query, often used to extract data like usernames or passwords.

: Only allow expected characters. For example, if a field is for a username, don't allow special characters like ' , ( , or * . : This is used to combine the results

If a website allows this input to run, an attacker could potentially: don't allow special characters like '