Давай разложим печенье поровну по тарелкам и сообща украсим стол.

Here давай means let’s.

In Russian, давай + 1st person plural verb is a very common way to suggest doing something together:

• Давай разложим... = Let’s lay out / divide...
• Давай украсим... = Let’s decorate...

It is informal and used when speaking to one person. If you are speaking to several people, or speaking politely, you would usually say давайте instead:

• Давайте разложим печенье...

Because both verbs are perfective.

In Russian, a perfective verb has no true present tense. Its non-past forms usually refer to the future:

• разложим = we will lay out / let’s lay out
• украсим = we will decorate / let’s decorate

So after давай, these forms naturally mean let’s do this and complete it.