TL;DR: pandas needs the xlrd package to read legacy .xls files and it is not installed. Run `pip install xlrd` and re-run your read_excel call. For modern .xlsx files install openpyxl instead.

## The error
```text
ImportError: Missing optional dependency 'xlrd'. Use pip or conda to install xlrd.
```

## Fix it
1. Install the Excel engine your file format needs:
   ```
   pip install xlrd
   ```
   Expected: install completes.
2. Re-run the read:
   ```python
   import pandas as pd
   df = pd.read_excel("your_file.xls")
   print(df.head())
   ```
   Expected: prints the first rows instead of raising.
3. If your file is .xlsx (not .xls), install openpyxl instead:
   ```
   pip install openpyxl
   ```

## When this applies
Use this when pd.read_excel raises the missing-xlrd message, which means the file is the legacy BIFF (.xls) format.

## When it does not apply
If the error names openpyxl, pyxlsb, or odf, install that engine instead. If the file will not open even with the engine installed, the file itself may be corrupt.

## Compatibility
All pandas versions. xlrd 2.x only reads .xls; it dropped .xlsx support, which is why .xlsx files need openpyxl.

## Root cause
pandas keeps Excel engines as optional dependencies to stay light. It picks the engine from the file extension, and .xls maps to xlrd, which is not installed by default.

## Edge cases
xlrd>=2.0 refuses .xlsx files with a clear error; do not fight it, use openpyxl. In locked-down environments, add xlrd to requirements so deploys do not regress.