The number refers the section in the system's reference manual in which the corresponding page resides, e.g. for command
date, (date(1), in your case), 1 refers to the "user commands" section, which can be accessed by executing the command
man 1 date.
man man. It lists all the standard sections:
The standard sections of the manual include:
1 User Commands
2 System Calls
3 C Library Functions
4 Devices and Special Files
5 File Formats and Conventions
6 Games et. Al.
8 System Administration tools and Deamons
Distributions customize the manual section to their specifics, which often include additional sections.