Do you remember when you used a magnifying glass to set a leaf on fire? The magnifying glass used the sun's light to make the concentrated beam of light that burned the leaf. That's how lasers work too!
Now, imagine we send a laser signal from space, it will travel a long way, and when it reaches Earth, we need something to be able to read the message it carries. This is where an "optical payload for lasercomm science" comes in.
The optical payload is like a really special telescope that can look at the laser signal and see what it's saying. It has special sensors that can pick up even the tiniest details of the laser signal, and then turn it into something our computers can understand, like a secret code.
So basically, an optical payload for lasercomm science is like a super-powered magnifying glass that helps us read messages sent through lasers in space.