This will be a motion to approve the Withdrawal Agreement agreed in December, and the extra codicil that was added in February. You are correct that it does not include the declaration on the ambitions for the future relationship.
If this motion passes, this will allow the government to introduce a Bill to implement the Withdrawal Agreement. It will also extend the date of withdrawal to 22 May, as agreed between the UK and the European Council last week.
On the subject of the political declaration, there is an amendment put forward by backbench Labour MPs that would allow the Commons to set the mandate for the future relationship. If this amendment passes, then it would mean a new political declaration would need to be written and agreed to (though that shouldn't be too controversial a process).
The full text of tomorrow's motion is available on the Order Paper.