Number of shots: Number of shots means how many times an algorithm is run to get a probability distribution of results.
Experiment is repeated: This means how many times an experiment is repeated with particular number of shots. Imagine, you repeated an experiment 100 times and you have 1,024 shots. In each experiement repetition some metrics is calculated based on probability distribution obtained from 1,024 shots (i.e. an algorithm repetitions). The metrics can be, for example expected value or any other metrics based on the results. Hence you have 100 values of the metric and you can do some statistics on them.
Overall, number of shots and experiment repetitions mean different things.
Default number of shots: IBM Q allows you to set different number of shots. Default value 1,024 is an arbitrary decision. There is no physical/technical reason for that value.