Piping curl output into grep
Curl detects that it is not outputting to a terminal, and shows you the Progress Meter. You can suppress the progress meter with -s.
The HTML data is indeed being sent to grep. However that page does not contain the text "Gene Symbol". Grep is case-sensitive (unless invoked with -i) and you are looking for "Gene symbol".
$ curl -s www.ncbi.nlm.nih.gov/gene/823951 | grep "Gene symbol" <dt class="noline"> Gene symbol </dt>You probably also want the next line of HTML, which you can make grep output with the -A option:
$ curl -s www.ncbi.nlm.nih.gov/gene/823951 | grep -A1 "Gene symbol" <dt class="noline"> Gene symbol </dt> <dd class="noline">AT3G47960</dd>See man curl and man grep for more information about these and other options.