mysqltuner: "MySQL started within last 24 hours - recommendations may be inaccurate." Why? I'm new to db management and db tweaking. I'm using mysqltuner to get recommendations for how to improve db performance. 

Mysql issues a warning message along with its recommendations:
"MySQL started within last 24 hours - recommendations may be inaccurate." 
It is true that I started MySQL within the last 24 hours, but why does this matter? What does the database do over a 24 hour period that will change the results of mysqltuner?
It is a basic caution that a MySQL instance running under 24 hours (1 day) may not have seen the entire query workload and thus the recommendations might be affected.

Of course, 24 hours doesn't guarantee the entire workload either, as you might have scripts that run weekly or monthly.

On a separate note, I would caution you on taking the advice of mysqltuner too strictly. Specifically don't make any changes that you don't fully understand just because the script says to do so.