In some ways the research you'll do for an MRes is a lot like the final dissertation required for an MA or MSc.
You'll select a topic, be assigned a supervisor and conduct an independent investigation before presenting your findings. As you'd expect, however, the research required by an MRes is much more extensive.
This might mean that you'll be expected to complete multiple research projects. This is more likely for technical or professional subjects that require training in different types of research.
Or, you might simply complete one large project. If so, you can expect this to be much longer than the dissertation for a taught degree.
The academic scope of your research may also be more demanding. You won't be judged by PhD standards (for which a substantial original contribution to knowledge is required). But you may be expected to be closer to this level than an equivalent MA or MSc dissertation.
The advantage of this is than an MRes really does provide a full academic research experience. Whereas the dissertation is a single (but significant) part of a taught Masters, your work on an MRes will make you a proficient and professional researcher.