Meaning of stratify parameter
stratify parameter will preserve the proportion of target as in original dataset, in the train and test datasets as well.

So if your original dataset df has target/label as [0,1,2] in the ratio say, 40:30:30. That is, for every 100 datasets, you can find 40, 30 and 30 observations of target 0,1 and 2 respectively.

Now when you split this original using the train_test_split(x,y,test_size=0.1,stratify=y), the  methods returns train and test datasets in the ratio of 90:10. Now in each of these datasets, the target/label data proportion is preserved as 40:30:30 for the classes [0,1,2].

Often, we want to preserve the dataset proportions for better prediction and reproduceability of results