Theoretical probability is what is expected to happen without conducting an experiment. In other words, it's a method to express what what is expected to happen based on the number of favorable outcomes and the number of total outcomes. For example, in the image provided, I attached a spinner.
The probability of spinning a 4 is 1/4 because there is 1 favorable outcome which is spinning a 4 and there are 4 possible outcomes which is spinning a 1, 2, 3, or 4. It's important to understand that each outcome is equally likely. This means in theory, there is a 1 in 4 chance we will spin a 4.
We can also find probability of given event by conducting an experiment. Experimental probability is when you actually conduct the experiment. In the second image provided, these could be possible results if you actually spun the spinner.