what's a tonal language, anyway?

It means that different tones in the oral expression of the language (or different accents in the written expression of the language) convey different meanings. For example:

bí (high tone) means "to deliver a baby"
bì (low tone) means "to throw up"; and
bi (mid tone) means "to ask"

... so if I want to ask my Yoruba subjects some questions without them thinking I'm about to give birth or get sick on them, I better get my accents and my tones right.