A memo is like a note your teacher or parent might write you, except it's from someone at work. A summer memo is a note that tells you important things you need to know about what's going to happen at work during the summer.
In the summer, some people take time off to go on vacation or be with their families, so the memo might tell you who will be away and who will be filling in for them. It might also tell you about any special events happening during the summer, and if you need to dress differently for work.
The memo might also remind you of any rules or policies that are especially important during summer, like making sure you drink enough water to stay hydrated in the hot weather. Overall, the summer memo is a little reminder of what to expect and how to stay safe and happy during the summer months at work.