Hello and welcome to the site. Thank you for your question.
A lowering of the tax code means you have less tax free income before your income is taxed.
If your tax code has moved from 707L to 387L it means your tax free income allowance has moved downwards from £7,070 to £3,870.
Has there been a change in your circumstances to bring the tax code down? The tax office should explain the reason why there is a change in your tax coding.
I hope this is helpful and answers your question.
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