A website's tagline should make it clear what the company does and what sets it apart from its competitors.
Literal
Website [gen-の] tagline [topic-は], that [gen-の] company [subject-が] what [obj-を] doing [and-て], competing-other-companies [with-と] where [subject-が] differs [question-のか] [subject-が] understandable [like-ような] thing [must-でなくてはならない].
Formal business-advice prose. The innermost cleft その企業が何を行っていて、競合他社とどこが違うのか ('what the company is doing and where it differs from competitors') is a compound interrogative clause modifying ような ('in a way that'). わかる here means 'be understandable / become clear.' ~でなくてはならない is a formal 'must be X' pattern — stronger and more written than ~でなければならない or ~でなければいけない. Tagline ('タグライン') is a loanword from marketing jargon.