Heroku free dyno hours [duplicate]

I want to host a website on Heroku which will be dynamic and will be interacting with a database for very simple queries. I want to use the free tier of Heroku which after credit card verification provides 1000 free dynos. I have read this at multiple places what this free dyno hour is but I am still not able to understand. Can someone, who has worked with Heroku, tell me what is the significance of 1000 dyno hours. Because 24*30=720 and this means that maximum possible consumption is less than what Heroku gives. I have an estimate of 4000 visitors to my website per day. Do I need to buy a Heroku plan or I am good to go with the free tier since I will be querying simple texts from MongoDB.