They are two words,
لا means No
May mean literally Escape,
When they come together "as they do mostly", they mean "Must"
لا بُدَّ أنْ / لا بُدَّ أنَّ / لا بُدَّ وأن : حتمًا أو مِن الضروريّ
Or they mean No Escape, when you talk about something should be done and you can't escape it. In another meaning "it can't be helped ".