"Mouth like a /gushing river/waterfall/". Incessant talking like water never stops flowing.

口若悬河：嘴里说的话像瀑布的水往下流一样。这个成语意思是说一个人的话很多，一说起话来就停不下来。

to say sb is good at braging!

I think it means someone is eloquent and talks a lot