Basically, a sampling frame is a complete list of all the members of the population that we wish to study.
To give an example, if we wish to study the underlying factors that cause patients to be admitted into hospital following an acute asthmatic attack in a given area (your population), then you would need to know the names of all the people in that area who have been admitted into hospital for this reason.
From a list of these names, you can then randomly select an appropriate number as representatives of the population (your sample) whom you can invite to take part in the research.
If we do not have such a sampling frame, then we are restricted to less satisfactory forms of samples which cannot be randomly selected because not all individuals within that population will have the same probability of being selected for the sample.
Thus we would find that our sample is a non-probability sample.
simple random sample random sample selecting participants