Could some one explain the ethymology please. Besides, is it simple present or imperative?
It is imperative. You're telling the person not to do an action, which typically has the prefix 'usi-' (singular) or 'msi-' (plural), both translating to the command 'do not'.
Following the prefix is the verb stem, which in this case is '-angalia' ('to look'). I'm not sure since I can't recall any examples to the contrary, but I believe all verb stems end in the syllable '-a'. In the negative imperative, this changes to '-e'.
For affirmative commands in the singular form (speaking to one person), the verb stem does not change, i.e. 'look at the camera' would be
'angalia (tazama) kamera'
and its negative imperative is '
Simple present tense ('you are not looking at the camera') would be 'huangalii kamera'
There are verbs that don't end in "a" but they're all loanwords, eg. kusahau, kurudi, kuharibu
Actually, I think I saw that kuketi is not a loanword but derives ultimately from a phrase like kukaa chini, but yeah, Bantu verbs otherwise end in -a.