Learn Spanish (Latin America): Sin embargo, lo importante es el cuajo. - However, the important thing is the rennet.

The neuter article lo combined with a masculine singular adjective creates an abstract noun phrase meaning the [adjective] thing or what is [adjective]. Here, lo importante means the important thing. You use this structure when referring to an abstract concept rather than a specific masculine or feminine noun.

Could I just say la cosa importante instead of lo importante?

Grammatically, yes, but lo importante sounds much more natural and native to Spanish speakers. Relying on la cosa for the thing is a very common habit for English speakers, so practicing the lo + adjective structure is a great way to make your Spanish sound more authentic.

Why do we use es instead of está here?

You use the verb ser (here as es) because this sentence is establishing an identity or equating two things: the important thing equals the rennet. Whenever you define what something is or link two noun phrases together in this way, you always use ser.

I know sin embargo means however. How is it different from pero?