Translation:You turn on
-washa is the causative form of -waka ("to burn, be in flames"). So, it means "to set in fire".
I wonder why DL is choosing the present indefinite tense "You turn on" to translate the -na- tense instead of present continuous "You are turning on"? While it's true that the -na- tense can often be close to English present indefinite, it's closer to present continuous and the -a- tense is MUCH closer to present indefinite. "Wawasha" -> "You turn on". It seems like using this translation for -na- is setting this course up for confusion when the -a- tense is introduced... Just my opinion.
It's definitely incomplete. This course is full of all of these random disembodied parts of sentences.
Unawasha kompyuta. = You turn on the computer.
Unawasha taa. = You turn on a light.
Unawasha mshumaa. = You light a candle.
Unawasha moto. = You light a fire.
There is a problem of relevance. This comes in an exercise about n/n class nouns!