To Cheer On, To Cheer For
する verb
おうえんする
どちらの力士を応援しているんだい？
Which sumo wrestler are you rooting for?
応援 is aid and root for, but for this verb the "root for" one is going to be more on the money. You can call this To root for, to cheer on or to cheer for.
The reading comes from 応援. It's also just the on'yomi readings you learned.