Okay I'll give you a few examples. An assessment-type of intervention for this patient would be to assess the urine output. Note the quantity and characteristics of the urine. All rationale means is reason. The rationale or reason for assessing urine output is so that you know how well the kidneys are functioning and the characteristics of the urine tells you many things. Is it dark or light? This is called concentration. Concentrated urine may mean he's not getting enough fluids. Is it clear or cloudy? Cloudy urine often indicates infection. Is there an odor? Again, foul-smelling urine indicates infection. There is a lot you can learn about a patient just by looking at their urine! Assessment is such a big part of what nurses do.
A therapeutic intervention may be giving the patient plenty of PO (by mouth) fluids, rationale is to help flush out the bladder. A teaching intervention is to teach the patient about the importance of drinking plenty of fluids, rationale is so that he may help be in control of his health. You see, it is pretty simple once you get the concept, just try not to make it more difficult than it really is
Your intervention section of your careplan tells people what you will be doing with your patient. The rationale behind your interventions says WHY you do those things, the why behind things is a very important part of being a nurse. Depending on your careplan book, many if not all of your rationales should be supported by research, this says that there are scientific reasons why nurses do what we do, even if they seem very simple to outsiders.
I know it seems confusing now, but by next semester you will have a better grasp of what the nursing process is all about, and by the time you graduate you will be using the nursing process constantly without even realizing it. Have fun in your first clinical!!