It's basically saying "insert one ass loads of code in one go, all written by AI".

It's a summary of code changes. The green + indicates how many lines were added, the red - how many were removed (though changed or moved lines are counted twice, once as removal for the old and once as addition for the new).

The OP is supposedly being asked to read and review thousands of lines of new code written by claude. Depending on the language and claude's writing style, those lines may be very dense and hard to read, but even if they aren't, reading code you haven't written is always more difficult than reading your own.