A JWT has three parts: header.payload.signature. Header and payload are Base64URL-encoded JSON. This page helps you decode them and understand what you are seeing.
Try the tool:
Use the full interactive tool here: Base64 Decoder
Example
JWT (example)
eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiaWF0IjoxNTE2MjM5MDIyfQ.s5c0uQ2iQ7lYfZ3QZr2fG4u6mVfK4o7lK2o0q0m0q0k
What you should decode
Decode the first two parts (header & payload). The signature is not Base64-decoded to JSON.
Related pages
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FAQ
Does decoding a JWT validate it?
No. Decoding only reveals the data. Validation requires verifying the signature with the correct secret/public key.