what does[:-20] and [-20:] mean during assignment when dealing with a dataset

Question

from sklearn import datasets
diabetes_dataset= datasets.load_diabetes()
x_train = diabetes_dataset.data[:-20]
y_train = diabetes_dataset.target[:-20] 
x_test = diabetes_dataset.data[-20:]
y_test = diabetes_dataset.target[-20:]

can someone explain to me what this section of code means: especially during assignment of the x and y train and x and y test what does[:-20] and [-20:] mean I have tried understanding but cannot