A Money Purchase Plan is a defined contribution plan like a Profit Sharing Plan. The main difference is that the contribution amount defined in the Plan document is a required contribution. The employer MUST contribute the required contribution amount on time or pay penalties to the IRS.
The amount of required contribution is usually defined in the document as a percentage of each employee's compensation. For example, the Plan could define the contribution amount as ten percent. This would mean that the employer must contribute ten percent of each employee's "eligible" compensation each year. The amount of compensation to use (the "eligible" compensation) in the calculation is also defined in the Plan document. If an employee's eligible compensation were $40,000 for a given year and the contribution was defined as ten percent, the employer would be required to contribute $4,000 to the Plan's trust account on behalf of that employee for that year.
Money Purchase Plans are mostly obsolete. Since similar contribution amounts can be achieved in a Profit Sharing Plan with discretionary contributions each year, there needs to be a special reason for the employer to commit to the requirements of a Money Purchase Plan.