'Must' as a noun describes something that should be done, read, seen etc. 'This book is a must'. 'Must' in your sentence refers to something which was stated before and should be done or is necessary to do because we live in times with a lot of competetion. (Today, people compete for everything jobs, markets, housing etc.)

It's a must = it's something you must have or must do (must = necessity) A fridge is a must in any home.