Traffic is the amount of data sent and received by visitors to a website. At present moment the most of web hosting services provide unlimited traffic. But there another point: server resources such as memory and CPU are limited. Standard of а server connection is 100 MB\sec, thus server should provide 34 TB per month. In real life shared web hosting services provide equal server configuration. Such configuration provides 10 TB at best. Usually server is used by 500 users, so middle traffic is 20 GB. If you need more 20 GB traffic per month, you have to place your website on other kind of web hosting, for example VPS.