Learn Portuguese (Portugal): O que fazes é bom para a relação. - What you do is good for the relationship.

When 'what' means 'the thing that' in a statement, you need to use o que. The word que on its own would usually just mean 'that' or act as a question word. Adding o turns it into 'the thing that' so it can act as the subject of the sentence.

Can I use aquilo que in this sentence instead?

Yes! Aquilo que and o que mean exactly the same thing. You could say Aquilo que fazes é bom para a relação and the meaning remains completely unchanged. O que is just a bit shorter and more common in daily conversation.

Since relação is a feminine noun, why does the sentence use the masculine word bom?

The adjective bom does not describe the relationship; it describes o que fazes (what you do). Because 'what you do' is an abstract idea rather than a specific masculine or feminine noun, Portuguese uses the default masculine singular form.

Why does this sentence use é instead of está?

The verb ser (which gives us é) is used for permanent characteristics or general facts. By using é, you are saying that this action is fundamentally good for the relationship as a general rule, not just temporarily in this exact moment.