The "forward-looking political declaration" or "political declaration setting out the framework for the future relationship between the UK and EU" is a document developed as part of the exit negotiations between the UK and the European council under Article 50 of the Lisbon treaty. It sets out the intentions of both sides in terms of negotiating position moving forward from the terms defined in the (legally binding) Withdrawal agreement. As such it is relatively short (under 30 pages), readable and bland. Further, since it's not legally binding, the EU has stated that it is still open to modifying it, for example to include an intention to retain a customs union, or to state really firmly that both sides don't want the "backstop clause" involving Northern Ireland to be activated, or to continue in perpetuity. The linked version above is the one rejected by the House of Commons on the 11th March 2019.
The Withdrawal Agreement is a massive international treaty between the UK (~300 pages in the linked version) intended to formally (and legally) describe the status of the UK-EU relationship at the moment of "exit day", whenever that is. This is the document which sets out the details of the transition period post Brexit (during which not much would change), sets out the system under which the final UK "divorce bill" of payments to the EU will be calculated and includes the contentious "Northern Irish backstop", which could, if no further agreement is reached, tie Northern Ireland into following EU rules exactly, while the rest of the UK diverged.
In terms of UK law, both these documents need to be ratified by both Houses to become law, however the conclusion of the EU council summit on the 21st of March 2019 only requires the Withdrawal agreement to be approved by the House of Commons before the end of March for exit day to be 22nd May. In principle, the current Withdrawal agreement isn't against Labour's stated Brexit plan, which deals with an alternative future relationship. Further, splitting the votes could negotiate the decision by the Speaker of the House of Commons, John Bercow, that MPs couldn't be asked to vote on fundamentally the same motion twice.