> redirects output to a file, overwriting the file.
>> redirects output to a file appending the redirected output at the end.
Standard output is represented in bash with number
1 and standard error is represented with number
2. They are separate, so the user can redirect them to different files.
2>&1 redirects the standard error to the standard output so they appear together and can be jointly redirected to a file. (Writing just
2>1 would redirect the standard error to a file called "1", not to standard output.)
In your case, you have a job whose output (both standard and error) is appended at the end of a log file (
cron.log) for later use.
For additional info, check the bash manual (section "Redirection"), this question, and this question.