In simple words
A transit is where a planet is in the sky now, compared with your birth chart. Here, today's Mercury makes a square (90°) to the spot where your rising sign was when you were born.
It is a time of pressure that pushes you to grow, and it lasts a day or two (longer when it is retrograde). Your Today page tells you when it is happening for you.
Love
A remark may be taken the wrong way. Explain gently.
Avoid sarcasm today.
Ask questions instead of guessing.
Work
Communication hiccups are likely. Confirm in writing.
Prepare more than usual for meetings.
Avoid overpromising.
Mood
A little scattered. Make a short list.
Nervous energy needs an outlet.
Take breaks from messages.
About the square
A square sets two planets ninety degrees apart. Their needs pull against each other and ask for action. Squares feel like tension or recurring challenge, and they are also where much of our drive and achievement comes from. Working with a square, rather than against it, often builds a remarkable strength.