It means to talk with in utter frustration and when someone rants, s/he won't stop talking until she's let go of all the anger or she totally said what she wanted to say.
for example i'm mad at my history teacher.
"OMFG, Ms. Antolin is sucha a freaken bitch, like today she just totally had a spazz attack and i was seriosuly pissed at her because she justb lamed me for everything and all i did was sat in the little corner and did nothing. and she completely yells at me! UGHH!!"
ahaha.. ye if someone rants, its probably gonna have a lot of vulgar words as well.