Pro is $20 a month. It is how you keep building after your free game: credits to claim every day, faster builds, and every tool.
Free vs Pro
Building: Free one game, Pro keep going as long as you have credits.
Credits: Free none, Pro a fresh batch to claim every day.
Build queue: Free normal, Pro priority (skip the line).
Tools: Free the core ones, Pro all of them.
You can upgrade any time from the "Upgrade to Pro" box, or from your avatar menu. Once you are on Pro, the sidebar shows you where to claim your daily credits.