it can also be that

It's just 'you know', with the meaning dependent on what follows it e.g. 'unajua ku-(verb)' is always 'you know how to -', while 'unajua kuwa/kwamba' is 'you know that -'. For the latter, you don't have to include 'kuwa/kwamba' e.g. 'unajua anakuja' is 'you know (that) he/she is coming"