"To keep a diary" means to regularly write down your thoughts, experiences, and events that happen to you in a personal book or journal, often on a daily basis. Full definition
Step 2: identify triggers and rewards of the behaviour you've chosen You can identify triggers and rewards by keeping a diary of the difficult behaviour for 1 - 2 weeks. (raisingchildren.net.au)
Keeping a diary for your dog can help your veterinarian diagnose a skin problem. (trupanion.com)
Keep a diary during that time of what happens to your body during elimination including how you feel. (ericamesirov.com)