yield data_matrix[stop - sequence_length:stop].values.reshape((-1, len(seq_cols)))

This line of code takes a subset of the data matrix, specifically the last 'sequence_length' rows before the index 'stop'. It then converts this subset into a numpy array and reshapes it into a matrix with 'len(seq_cols)' columns and an unknown number of rows (determined by the number of rows in the subset divided by the number of columns in the reshaped matrix). This reshaped matrix represents the input data for a model that predicts the next value in the time series given the previous 'sequence_length' values.