What is the difference between Rubisco and Rubisco activase

What are their functions and what exactly are they? What is the difference between them?
(simple, uncomplicated explanation please)

RuBisCo is an enzyme used in carbon fixation (the "dark reactions" of photosynthesis). It adds a molecule of carbon dioxide to a molecule of the phosphorylated carbohydrate ribulose bisphosphate (also called RuBP) at the beginning of the Calvin cycle.

The cool thing about enzyme names is that they tell you what the enzyme does. Rubisco is an abbreviated form of "RibUlose BISphosphate Carboxylase/Oxygenase", which tells you that the enzyme adds carbon dioxide and oxygen to RuBP. So, Rubisco activase activates Rubisco.

Rubisco activase actually works by allowing the formation of another molecule that sits in Rubisco's active site, making it easier for RuBP to leave the active site. So, Rubisco activase keeps Rubisco catalytically active.