水分補給 literally means to supply water (水分 - water, 補給 - supply). The term is used to describe the process of hydration (meaning to drink water or other beverage, not the chemical process).

It has been a hot summer in Japan, and today (June 25) many cities’ highest temperatures are near 40 degree celcius (over 100 degree fahrenheit). In a day of such high temprature, it is critical to stay hydrated, thus many accounts, owned by governments, by restaurants, by stores, etc., are tweeting to remind their readers to drink water, causing the tag 水分補給 to trend.