"I must read"
As a noun it means 'rule' or 'condition' (kwa sharti moja: 'on one condition'). But in this construction, one could think of it as 'it is mandatory' (that I read), i.e. 'I must read'. Some more examples:
Mtu sharti afanye kazi kila siku: 'a person must work every day'
Ni sharti nikusaidie: 'I have to help you'