Data usage is the amount of data consumed when browsing websites, watching videos, and using apps on your smartphone. In Japan, it's colloquially called "giga" - as in "I'm out of giga."
Measured in MB and GB (1GB = 1,024MB). 1GB roughly equals 500,000 LINE text messages, 3,000 web pages, or 2 hours of standard YouTube video.
Mobile plans have monthly data caps. Exceeding the limit triggers speed throttling, making video and downloads extremely slow until the next month's reset.
Save data by using Wi-Fi whenever possible, lowering video quality settings, setting app updates to Wi-Fi only, and restricting background data usage.