Crontab is a Unix-based utility that allows users to schedule tasks or commands to run automatically at specified intervals on a Unix or Linux system. It stands for "cron table," where "cron" is the name of the daemon (a background process) that executes scheduled tasks, and "table" refers to the list of commands and their corresponding schedules. Crontab uses a specific syntax for specifying the schedule or time intervals for tasks to run. The general format of a crontab entry is as follows:

* * * * * command-to-be-executed
- - - - -
| | | | |
| | | | ----- Day of week (0 - 7) (Sunday is both 0 and 7)
| | | ------- Month (1 - 12)
| | --------- Day of month (1 - 31)
| ----------- Hour (0 - 23)
------------- Minute (0 - 59)

Each field represents a time unit, and you can specify values or ranges for each field to define when a command should be executed. Asterisks (*) represent all possible values, and you can use commas (,) to specify multiple values or hyphens (-) to define a range of values. For example, the following crontab entry will execute the `my-command` every day at 3:30 AM:

30 3 * * * my-command

You can use the `crontab` command with various options to manage crontab entries. For example, `crontab -e` opens the crontab file for editing, `crontab -l` displays the current crontab entries, and `crontab -r` removes the crontab entries for the current user. Note: Crontab entries are specific to each user, and you need appropriate permissions to modify the crontab for another user. Additionally, changes to the crontab usually take effect immediately without requiring a system restart. However, it"s a good practice to double-check your crontab entries and test them thoroughly before deploying them on a production system.