This comes from the exported wallet. Example:
secret-extended-key-test1qwhxeyunqqqqpqxvgxm6g3mcygghq804xyfm7hyqf07lgdrr9smx97p4efs46ptkvtqhfph9rz03nkfpaph7welf89ejqcq32s46c96caxwdvkc8dn4s4jz0ndy8pyrdundpetf7rkh9u2hjr45luxu0z675p6ymecnjpnqdr9vyelckk8zpf2wr5mvl062zt8ewjsk89wkc96lzmw09rzv07fxjfpr0q603lcf2vngqfj42jhdnpujcd5rgfj6qzj78cf9l83udhrqv7ydq5

This is the full viewing key, which can be generated with the tool "secret key to full viewing key". Example:
zviewtestsapling1qwhxeyunqqqqpqxvgxm6g3mcygghq804xyfm7hyqf07lgdrr9smx97p4efs46ptkvfeynd9qzze57lws8yu5l8s5698lh74gv4akujmwz0jdw4um3992hrm6ke67xwdsc9ptscvwsfvdjrajg5pwtg5axkpmwkrxh5wwdvvur9vyelckk8zpf2wr5mvl062zt8ewjsk89wkc96lzmw09rzv07fxjfpr0q603lcf2vngqfj42jhdnpujcd5rgfj6qzj78cf9l83udhrq6la7tg

This comes from the exported wallet. Example:
secret-extended-key-test1qwhxeyunqqqqpqxvgxm6g3mcygghq804xyfm7hyqf07lgdrr9smx97p4efs46ptkvtqhfph9rz03nkfpaph7welf89ejqcq32s46c96caxwdvkc8dn4s4jz0ndy8pyrdundpetf7rkh9u2hjr45luxu0z675p6ymecnjpnqdr9vyelckk8zpf2wr5mvl062zt8ewjsk89wkc96lzmw09rzv07fxjfpr0q603lcf2vngqfj42jhdnpujcd5rgfj6qzj78cf9l83udhrqv7ydq5

What is this about?

Zcash, since Sapling, supports the concept of diversified addresses. These are addresses derived from the same secret key, and are unlinkable among each other. The official Zcash client supports these addresses, but cannot generate them currently. This tool bridges this gap and can generate multiple addresses for the same key.

Additionally, Zcash supports the concept of full viewing keys, which is a key that allows to decrypt encrypted notes and generate addresses, but not create proofs or spend notes. Using these keys on an online PC to receive ZEC is much safer. Your secret key can remain in a cold storage or a hardware wallet. This tool allows you to derive your full viewing key from your secret key, and generate addresses from this key.

A video demonstrating the usage of this tool and its interaction with the Zcash client is available here.

This tool is an experiment in running sapling-crypto in the browser. It uses work by Stefan Deml, which modified the ZKP library Bellman to be WebAssembly compatible.

For safety reasons and this being an expermintal, I've disabled the Mainnet mode, so you'll have to put some effort in order to enable it.

What can I do with it now?

I encourage you to use the built-in testnet key, or use your own, to generate addresses and transact on the Zcash testnet, to better get a feeling of its usage!

If you have any questions, feel free to contact me at kobigurk@gmail.com.